Flaperons are when you use the ailerons as flaps and ALSO as ailerons. Some also use them as spoilers by deflecting them upward.

Here is a handy glossary of RC words/terms: http://www.rcuniverse.com/community/glossary.cfm It can also be gotten to through the "RC COMMUNITY" tab at the top of this page.

Oh, how much they help is subjective and varies from plane to plane and by the amount used. They can actually hurt the performance of some planes and can make tip stalls occur more readily.