For those who may not be familiar with the lingo, let's define what we mean by a digital SLR (or DSLR).
A digital single lens reflex camera includes a mirror that flips up when the picture is taken. The mirror and an internal prism allow you to frame your shots through the attached lens. The mirror flips up to reveal the sensor when you press the shutter button. Additionally, DSLRs allow you to use interchangeable lenses.
Don't confuse DSLRs with the new, fast-growing category of hybrid cameras. These are fine cameras that also offer interchangeable lens systems, but they use an electronic viewfinder instead of the mirror to compose your image.
The DSLR camera world is divided into full-frame sensors and reduced-size APS-C sensors.
Cameras with full-frame sensors start at about $2,000. They appeal to pros and advanced amateurs because they offer the highest image quality. These cameras also usually capture more frames per second, which is critical in sports and wildlife photography. Top DSLRs are also more weatherproof than entry-level DSLRs.
A full-frame sensor is the same size as traditional 35mm film - 24 x 36mm. Unlike cameras with smaller sensor sizes, there is no magnification factor when changing lenses. A 50mm lens behaves like a 50mm lens.
The smaller APS-C sensors in entry-level and intermediate DSLRs translate to lighter, more compact cameras. They start at about $700. They might not quite match the build or image quality of their full-frame counterparts, but they are still very capable cameras. They can produce stunning 16 x 20 prints or better and gorgeous HD video footage that's worlds ahead of point-and-shoots and smartphone cameras.