You should know Alexander Calder. He was a sculptor and painter whose unmistakeable works have become ubiquitous at art shows. Many galleries hold pieces by the prolific artist--and his work is popular now. The gallery has a great bio on the artist at their website.
No matter your level of art knowledge you will, if you visit museums, art shows, galleries, eventually run into artists you don't know . Sam Francis was a new artist to me as was Theodoros Stamos.
Francis is known for his color. Was he an abstract expressionist? Was he a color field artist? I really cannot say. Ask an art historian. The piece below seems more abstract expressionist but, really, how much does what "school" an artist belonged to matter? Did it matter to them?