Resolution - This is the number of megapixels that the camera has. All modern cameras have plenty of megapixels (MP). Even if you print at large sizes, 10-12 MP is more than plenty. A higher resolution (more megapixels) is not always better as it can introduce more noise and degrade image quality.
Sensor Size and Focal Length Multiplier - The sensor size is what sets digital SLRs apart from point and shoot digital cameras. DSLRs come in several sensor sizes including the micro Four Thirds system with a 2x focal length multiplier, APS-C with a 1.5x or 1.6x multiplier, APS-H with a 1.3x multiplier, or full frame (35mm) with a 1x multiplier. As you can see, the multiplier increases as sensor size decreases. The multiplier is the factor at which you multiply your focal length used to obtain a 35mm equivalent.
How is this useful? This means that DSLRs with slightly smaller sensors can use cheaper and lighter lenses to obtain equivalent 35mm focal lengths!
Lens Mount - Each manufacturer has their own lens mount and can only utilize lenses of this particular mount unless an adapter is used. For example, Canon cameras use the EF and EF-S mount, Nikon cameras use the F mount, Olympus and Panasonic use the Four Thirds Mount, and Sony uses the Alpha A mount.
ISO Capabilities - ISO is how sensitive the camera is to light. Better cameras perform better in low-light as they have a better signal-to-noise ratio. One of the factors what really sets modern SLRs apart is not their resolution but their high ISO capability.
Autofocus - Advanced and multi-point autofocus capabilities are utilized when shooting fast-action sports where focus on fast-moving objects is critical. The more focus points a camera has, the more expensive it will be.
Frame Rate - Frame rate is recorded in frames per second (fps). It is the number of consecutive images that can be taken in one second. A higher rate will allow you to take many images in a row creating a sequence. This is especially useful in fast action moving sports. By taking a burst of images, you increase the success rate of number of keeper images.
X-Sync Speed - X-sync speed or flash sync speed is the fastest shutter speed that the camera can use to fully expose the sensor when flash is used. Most cameras have a flash sync speed value of 1/160 to 1/250 of a second. This is particularly useful to minimize ambient light when using off-camera flash.