It's well-known that C-3PO is a protocol droid.
R2-D2 is an astromech.
What is BB-8 and what does that function entail?
This is likely to be answerable now based on marketing material or toy releases, but please note per meta discussion, it is against policy to use information from The Force Awakens before its official release. And the answer should be spoiler tagged if it contains any plot elements or other spoilerish information.