Okay, so I didn't cover "top" all that well in this posting. Let's try again ...
Top is a combed preparation -- manually, we use handcombs for this, and pull the top off once it is combed. Large scale combing equipment is still a mystery to me. It's expensive -- it must be -- and maybe even takes longer than carding, because the mills that do both, charge more for combed fiber than for carded. The fibers are all parallel to each other. Because of this, it is very good for a worsted-style of spinning.
Roving is a carded preparation, and technically speaking is also supposed to have some twist in it. On handcards you roll the fiber into a rolag, which adds some twist to them, but on a drum carder, typically the carding doesn't twist the fibers, just brushes them into a neat batt. So, batts can appear to be 'top', but they aren't combed, just brushed into a semi-organized state that can be spun almost worsted.
The abso-positively-lutely exact term for roving without twist is sliver, at least it seems to be what Ashford uses to describe their carded but untwisted wool. So batts are very similar to "sliver", just much wider and flatter ;-)
Because roving and rolags are carded and have some twist of the fibers in them, they are great for trapping air within the spinning, which makes for a good woolen-spun yarn.
(based on a posting by me on Spin-List, 17May07)