The simplest is to compare them side-by-side. This will give you specification differences. Then you have to divided the differences into two groups: Ones that matter to you and ones that do not.

For example, some DSLRs have a sync-port. Some people look for it, some do not. A top shutter-speed of 1/8000 vs 1/4000 does not matter to everyone either.

Some differences you will encounter:

• Sensor size (Affects image quality, depth-of-field, field-of-view of your lenses)
• Lens mount (Decides which lens you can use, which ones will autofocus)
• Stabilization (Determines if all lenses get stabilized or not)
• Maximum ISO (This an indication of low-light performance but not directly correlated since not all ISOs show the same image quality)
• Maximum Shutter-Speed (How fast you can capture motion without using a flash as primary light source)
• Viewfinder coverage (Either you can precisely frame in-camera or not)
• Viewfinder Size (Affects comfort, perception of focus and fine-details)
• Number of control-dials (Efficiency of use)
• Weather sealing (Usable in rain, snow, sandstorms, etc - Note: requires weather-sealed lens too)
• Metering modes
• Depth-of-field preview
• Maximum continuous drive
• Autofocus points
• Weight
• Size
• Price
• Much much more... the database at Neocamera has over 180 data points per camera, about 50 or so are exposed in the Camera Finder which you can use to find cameras that match specifications you need.