Digital SLRs provide tremendous flexibility for photographers who need to tackle a wide variety of photo assignments. The key feature is the removable lens. Major camera manufacturers such as Nikon and Canon provide you with dozens of lens choices for your DSLR.
Sports and nature photographers may lean toward powerful zooms that bring the action in close. Special event shooters will want a high-quality wide-angle lens for working in tight quarters. Portrait photographers need moderate telephotos with wide apertures so they can soften the background. Regardless of how you want to use your DSLR, there's a perfect lens for you.
In this section I'll focus on a few of the key features that distinguish these types of professional cameras from compact and advanced amateur models.

Electronic flashes

Most compact camera shooters, and even many advanced amateurs, live and die by the flashes that are built into their cameras. As you get more serious about your photography, you should consider using at least one external flash unit.
The most basic application is mounting a single flash in the hotshoe of your DSLR or advanced amateur camera. This alone will improve your shots, because you'll have moved the light source (the flash) farther away from the picture-taking lens. By doing so, you'll reduce the effect of red eye and move unsightly shadows lower behind the subject.
You also have the option of using a dedicated flash cord to extend the distance between flash and camera lens. Wedding photographers often use a bracket to position the flash exactly where they want it. The effects of red eye are completely eliminated when using this type of rig.
Wireless flash control is a great alternative, especially when you want to use two or more flash units to light a composition. Typically, you mount a wireless controller in the hotshoe of the camera, then position your flashes on light stands. When you trip the camera shutter, the wireless controller sends out a signal telling the flash units when to fire and for how long. This amazing system enables you to create sophisticated lighting setups without cumbersome wiring.
Many DSLRs include a pop-up flash on the camera body. This function may come in handy in a pinch, but external flash units are an option worth considering if you're serious about this type of photography.

Image sensors

Instead of film, digital cameras record light with solid-state devices called image sensors. I'm going to spend a little time explaining some of the differences commonly found in these components. If this type of discussion gives you a techno-headache, you can read through my image sensor rules of thumb in the next paragraph and skip the rest of the discussion.
Bigger image sensors (in physical dimensions) generally produce better image quality. That's one of the reasons why digital SLRs outperform compacts—they have more real estate to record pixel information. Speaking of pixels, the more megapixels your image sensor supports, the higher the resolution of the photo will be, and therefore the bigger the print it can produce. So, as a rule, 3-megapixel cameras are great for snapshots, but you really need a 5-megapixel or greater sensor for enlargements. That said, keep in mind that the image sensor is only part of the quality equation. The camera's optics and electronics play major roles too.
If you want to know more about why these rules apply, here's a short course in image sensor technology.
The most common sensors are charge-coupled devices (CCDs). However, many cameras, such as the Canon SLRs, are now employing complementary metal oxide semiconductor (CMOS) sensors, which share many of the same attributes of CCD types but use less energy. Another type of sensor, called the Foveon X3, is the current choice for Sigma SLRs. The Foveon sensor has a much different design compared to its CCD and CMOS brethren. It actually uses three separate layers of pixel sensors embedded in silicon, whereas CCD and CMOS sensors have a single layer.
Image sensors also vary in their dimensions. Many entry-level digital SLRs use sensors that are referred to as APS in size. The term APS comes from the alternate 24mm film format (Advanced Photo System) that was introduced in the 1990s but never really gained traction. The label survives because many of today's digital SLRs have image sensors approximately the same size as an APS film frame (roughly 15mm x 23mm). Because the proportions of these APS sensors are smaller than those of 35mm film (24mm x 36mm), cameras containing them have increased image magnification when traditional 35mm lenses are mounted on the body. Typically, this increase is around 1.6x.
Some digital SLRs employ a four-thirds image sensor. The major proponent of this system is Olympus. The term four-thirds refers to the proportions of the image sensor, producing images that are 4:3 in dimension. Current four-thirds sensors by Olympus are approximately 13mm x 17mm—smaller than APS-sized sensors, but larger than those found in most point-and-shoots. At the other extreme are the pro-level full-frame SLRs with 24mm x 36mm sensors (the same dimensions as 35mm film).
Instead of physical size, however, most people refer to image sensors by how many pixels (picture elements) they support. The term megapixel means just that: a million pixels. So instead of saying, "I just bought a camera with a sensor that supports 5,000,000 pixels," you can say, "I just bought a 5megapixel camera."
Consumer cameras currently range in capacity from 2 to 8 megapixels. Pro cameras have sensors as large as 14 megapixels. Generally speaking, you want at least 3 megapixels for snap-shooting and vacation pictures. The more megapixels your camera has, the bigger the prints you can make. Three-megapixel cameras, for example, can produce quality prints at up to 8" x 10";.
Advanced amateurs and pros need more pixel-power than vacation shooters. Having a 6-, 8-, or 12-megapixel image provides you with more options when you process the image on the computer and print it out. You can, for example, push the pixels closer together (increasing the "pixels per inch" setting) to create very smooth tones in the photograph, rivaling the images produced by high-quality film cameras.
More pixels also enable you to crop the original photo, maybe choosing just the center portion of the picture, and still have enough image information to make a high-quality enlargement.
A hefty-megapixel image sensor, however, doesn't ensure amazing photo quality. Other aspects of the camera's optics and electronics play important roles too. For example, a 6-megapixel sensor in a compact camera will be in the neighborhood of 7mm x 9mm in physical size, but a 6-megapixel sensor in a digital SLR will be 15mm x 23mm or larger. That means that each of the photosites (photosensitive diodes that collect one pixel's worth of light) on the DSLR's sensor is physically bigger. These bigger photosites collect more light and result in better image quality and reduced digital noise.
In the end, the best way to think about image sensors is the same way you think about the engine in your car: yes, it's vital to the car's performance, but there are many other factors that contribute to a good ride. And don't forget, the driver has something to do with it too.

Optics

Since you have so many lenses to choose from with a DSLR, where do you start? Regardless of your specialty, all photographers need one or two "bread and butter" optics for everyday use.
The most essential lens is the moderate wide-angle to telephoto zoom. The Nikon VR 24mm to 120mm and the Canon IS 17mm to 85mm zooms are good examples. They both range from substantial wide angle to moderate telephoto with 5x magnification. Both incorporate image stabilization technologies to reduce the effect of camera shake in low-light conditions. And with either, you can go out for a day of shooting with just that lens and be ready for most situations you'll encounter.
When shopping for a lens for your DSLR, keep in mind that you might have to factor image magnification into the equation (see the earlier discussion of zoom lenses under "Advanced Amateur Cameras" for more information). The Canon 20D, for example, has a 1.6x image magnification, which means that a 17–85mm zoom lens will become a 27–136mm lens when mounted on the 20D.
Finally, always keep portability in mind when lens shopping. You can spend hundreds or even thousands of dollars on a wide-aperture lens with an impressive zooming range, but if it's too heavy to cart around, or won't fit in your camera bag, you've defeated your primary purpose: to buy a lens that you like to shoot with and will have with you when you need it.

WiFi image transfer

Nikon made a splash at the 2004 PMA show with its WT-1 WiFi adapter for the Nikon D2H DSLR. This adapter enabled photographers to "send" their images via 802.11 wireless net works, eliminating the need to physically connect the camera, or the memory card, to a computer.
WiFi technology has been around for some time and is typically used to enable Internet connectivity in coffee shops, airports, and businesses that have "hotspot" capability. Soon, sending pictures from your camera might be as easy as sending email from your computer. Kodak has already announced a consumer camera with this technology built in, and more are sure to follow.