Battle Slugs is essentially a stylized version of the old Chinese game Go. You start with two different colors of playing pieces which jump around the board in different ways. Jump one space to produce a cloned slug or two spaces to move the slug you're controlling. When you land next to a piece of the opposite color, it changes color.
Those are basically the only rules, but like Go, the strategy gets complicated quickly. You want to try and claim space on the board (the one you're playing at least -- there are 25 different levels in the game), but you have to do so carefully, because overextending can make trouble quickly. The game's actually pretty hard -- I had trouble mastering even the first level consistently.
Battle Slugs was originally a Flash game, but you can't tell from the presentation on iOS. While the graphics can be somewhat crude, they are colorful, and bring a lot of personality to the title. Loading times are a little long (though that's of course between matches, not during), and the iTunes reviews have some complaints about the game's performance on older phones, so beware of that.
But there's a lite version to try out first if you want to make sure you like it, and the full version (which is Game Center enabled as well) is only 99 cents for a universal build anyway. If you like the strategy of Go and don't mind some cartoony characters, Battle Slugs could tease your mind for a while.