The Evan's Gambit is an aggressive variation of the Giuoco Piano. For those players that like to play the Giuoco Piano I highly recommend playing the Evan's Gambit. Much like other gambits, the Evan's Gambit gives up an early pawn in exchange for rapid development and a lead in center control. White actually scores better with the Evan's Gambit than not playing the gambit and following the main lines of the Giuoco Piano which is why I recommend playing it.
After black takes the pawn on b4, white will always continue with c3, follwed by d4. The difference will depend on where the black bishop retreats to. The main line is for black to retreat back to a5 but you may also see him move back to c5, his originally square before he took the pawn, or back to e7 which seems on the surface to be a pretty safe square for black's bishop.