The list BigB points to is a good one.
Ultimately, there's no hard and fast distinction between "base malts" and "specialty grains", but one division commonly used diastatic power. Base malts possess enzymes that, when activated at the right temperature (via mashing), can convert starches (which are inaccessible to the yeast) to sugars (which are yeast food). Specialty malts don't typically possess these enzymes, and need to be mashed in the presence base malts or their starches will stay unconverted.
This is complicated by the fact that "specialty grains" are sometimes used to refer to any grain that is steeped by extract brewers. Specialty grains tend to be roasted or crystalized, and likewise can have some of their flavor and color extracted from them just by soaking them in hot water. This isn't perfect because you certainly could steep a base grain. It doesn't do terribly much for you because base grains tend to be very light, but there are some benefits in particular circumstances.
The list BigB pointed to is a good general reference, but don't get too hung up on the categories. Just because something is marked "need to mash" doesn't mean that you will never ever ever see it steeped, and just because something is marked as "not needing to be mashed" doesn't mean that you will get equivalent results from steeping as you would from mashing.
