They're functionally different. The slots and dimples match the shape of the miniature's attachment point—a wide tab for slots and pins for dimples. Plain bases are intended for flat-based miniatures.
Bases can sometimes be used with the "wrong" miniature, though it's usually not worth the effort, and not all miniatures are good candidates for the needed modifications. Plain can be used for any kind of mini, assuming you're willing to cut off the tab or pins and you're confident that the remaining bottom surface of the miniature is enough to stay glued to the base. Slotted and dimpled bases can be used to mount flat-bottomed (or modified) minis, but provide less surface area for glue.
For purchasing, you should buy the base that matches your miniatures. For the average RPG miniature that will be Plain, since most RPG miniatures are made to stand without bases, and a plain base provides the most contact area between plastic base and miniature base for solidly gluing them together.
I lied, just one picture. Here's a mini with a tab beside the slotted base it will be mounted on:
These are common in wargaming because the tab-and-slot provides a lot of surface area for glue, making for a durable connection suitable for heavy handling. If your mini doesn't look like this, you don't need slots, but if it does, you do!