A solid rhythm guitarist is at the heart of many great bands and is sometimes expected to provide backup vocals. The rhythm guitarist can work closely with other members of the band and is often responsible for ensuring a cohesive sound.
A lead guitarist is expected to provide flare in both the music and on stage presence. They will usually write or improvise every guitar solo and are sometimes responsibly for writing other lead parts and fills.
Some bands can get away with a single guitarist whilst others alternate the lead and rhythm guitarists between songs.