Actually Rupert is quite right. Chipmunks are a type of smaller squirrels. I like them a lot actually. Don't see them as much as I used to when I was a kid though. Lots of gray squirrels here but not too many of the little chipmunks.
http://en.wikipedia.org/wiki/Chipmunk
The solution for diverting the squirrels and chipmunks from the bird food is peanut butter. If you put some PB on a dried corn cob and cover it with a bit of bird seed, hang it up below the bird seed feeder or close by most of the time they'll head straight for it and leave the bird seed well alone. (A bunch of raisins is also good stuck on.) They just love the peanut butter and it helps them put weight on when they need it besides. I do that, also leave out a milk container of raw nuts up somewhere where only the squirrels are likely to be once in a while.