A soloist is an individual musician that is being featured in some way. Like a guitar solo in a rock song, the soloist might be heard over the rest of the orchestra for a brief solo while the rest of the “band” stays out of the way and supports the soloist.
A concerto is written so that the entire piece features one soloist out in front of the orchestra. A concerto is intended to show off all aspects of the soloist’s technical and expressive mastery of his or her instrument, from fast pyrotechnics (referred to as “virtuosity”) to slow, sweet, and lyrically sensitive musicianship.
Some compositions, such as Beethoven’s Triple Concerto for Violin, Cello, and Piano, feature multiple soloists in front of the orchestra. Others, like Handel’s Messiah, have singers featured in front of the orchestra who sing solos, duets, trios, and quartets throughout the work.