i'm quite passionate about polyominoes of all shapes and sizes, due to working with blokus and pentomino codes.
a polyomino is a geometric figure formed by joining squares of equal size, edge to edge.
there are only a few practical polyomino forms:
these are the only variants you will see in a practical environment. 'hexominoes' are the variant for six pieces, and is the last available variant you can tile each individual form of into a square. there are 35 versions of this, so, not exactly practical for use. they aren't coded to letters, either, so why even bother.
there's also a 'monomino' and 'domino', which each only have one variant - a single square or two squares adjacent - though a domino can be oriented in 2 different ways
a tetris game, as it is named for, is played with a set of 7 tetrominoes.
blokus, one of my favorite things (i say 'thing' and not 'game' because i don't play it i use the pieces for stop motion animation) uses a set of 21 different pieces, including every free tromino, tetromino, pentomino, and additionally bears a monomino and domino. this makes for a total of 84 pieces in a set, because there are four colors.