Originaly, Selector (Selecta) is the one who selects and passes the records (vinyls) to the guy that is playing them on the soundsystem. It originated in reggae/danchall music, but nowdays the word is used to describe a DJ (wich is wrong - but accepted)
Boom Selecta is synonymous with DJ or disk jockey. This can be any person at any level who is choosing the music for a party, club, or social event. In proper English it would be Boom Selector, however Selecta sounds better. In other words, a Boom Selecta is someone who selects mad booms.