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)
Used in many songs.. Rewind Selecta, Pull Up Selecta, Bo Selecta,...
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.