A helicase is an enzyme which aids the unwinding of the double helix during DNA replication, thus exposing the bases so they can be copied. In order to drive the unwinding of the double strand, a helicase hydrolyzes ATP. This is due to the fact that splitting the strands requires breaking of the hydrogen bonds between the DNA bases.
There are usually several different types of helicases which are specialised for different roles such as repair, replication, transcription termination, RNA splicing, and RNA editing and recombination. There are DNA and RNA helicases.