Eukaryotic Rad51 and its related family members are homologous to the archaeal RadA and bacterial RecA recombinases.  Rad51 is highly conserved from yeast to humans.  It has a key function in the recombinational repair of DNA damages, particularly double-strand damages such as double-strand breaks.  In humans, over- or under-expression of Rad51 occurs in a wide variety of cancers.  
During meiosis Rad51 interacts with another recombinase, Dmc1, to form a presynaptic filament that is an intermediate in homologous recombination.  Dmc1 function appears to be limited to meiotic recombination.  Like Rad51, Dmc1 is homologous to bacterial RecA.