Nothing. An electric guitar is used as a rhythm and ALSO a lead guitar. However, the difference between rhythm and lead guitar is just the guitar riffs that are played. Rhythm essentially keeps the rhythm/basic notes of the song while the lead guitar do a fancy lead riff and/or solo overtop of the rhythm guitar.

Guitar, Violin, Bass Guitar, Rhythm Guitar, Acoustic Guitar, Piano

Yes. They occupy different parts of the music; the rhythm guitar is generally part of the rhythm section and the keyboard is generally part of the melody section.

Guitar, Violin, Bass Guitar, Rhythm Guitar, Acoustic Guitar, Piano

James Hetfield - Rhythm Guitar

Kirk Hammett - Lead Guitar