The basic specification of all high-end smartphones is pretty standard: a 1GHz ARM processor; a 3.5in. to 4in. display with a resolution of about 480 by 800 pixels; 3D graphics acceleration; and around 512MB of RAM.
Internal flash memory varies widely though, from 512MB up to 32GB in Apple's iPhone 4. Android devices from HTC currently have 512MB of internal flash, while Samsung goes up to 16GB. Expansion via external SD cards is generally available for Android phones.
Another distinguishing feature is the camera: some smartphones have a front-facing camera (for making video calls) as well as the usual rear-mounted unit. An LED flash may or may not be present. All this adds up to big differences in picture quality.
What's not evident from the smartphone manufacturers' specifications is the fact that there are significant performance differences. Fortunately, these are clearly measurable.