The Portsmouth Gambit aims at pulling the troublesome c5-pawn out of the center. You hope to de-fang the Sicilian in this way. Black has two ways to accept this gambit and has played several moves when declining it.
If Black wants to accept the gambit he can do so by 3...cxd4 or 3...Nxb4. Declined lines can start with moves like 3...b6, 3...e6, 3...d5, 3...d6, 3...e5 or 3...Qc7.
Both Accepted Lines give White a development lead with the center ripe for opening at the right moment. The 3...Nxb4 line in particular has shades of the Evans Gambit about it.