In Italian, the word solo means “alone.”
A solo is a piece of music that someone shares by himself or herself. A solo can still be a solo if there is someone (or some people–even a whole orchestra!) accompanying the person who plays the solo. We call the person who plays a solo a “soloist.”
Singers and instrumentalists who sing by themselves are examples of soloists. David Archuleta, Jenny Oaks Baker, Louis Armstrong, Joshua Bell, Yo-yo Ma, James Galway, Yolanda Kondonassis, Peter Hollens, William Joseph, Daniel Barenboim, Lang Lang, Hélène Grimaud, Nat King Cole, Billie Holliday, Bing Crosby, Judy Garland, Frank Sinatra, Julie Andrews, etc.
In French, the word ensemble means “together.”
An ensemble is a group of people who perform together, such as a duet, a trio, a quartet, a quintet, a choir, an orchestra, a symphony orchestra, and a band.
Examples of famous ensembles are: The Kings Singers, Academy of Ancient Music, Vienna Boys Choir, the Mormon Tabernacle Choir, Academy of St. Martin in the Fields, Canadian Brass, The Beatles, The Jackson Five, etc.
Most major cities in the world have at least one orchestra. There is the San Francisco Symphony, the Los Angeles Philharmonic, Boston Symphony Orchestra, Boston Pops Orchestra, St. Paul Chamber Orchestra, London Symphony Orchestra, Royal Concertgebouw Orchestra, Berlin Philharmonic, Czech Philharmonic, etc.
Here are links to ideas for teaching about solos and ensembles: