A maru is the Japanese symbol for "right" or "correct." It is a circle. Often used in children's anime and other mass-market Japanese TV. Contrast with batsu, or "X", which is used to mean "wrong." An upward-pointing equilateral triangle is used to mean "maybe" or "sometimes" in the same context. I'm not sure what the square, L1, L2, R1, R2, Select, or Start mean; Sony probably chose them for different reasons.
Thanks to fhayashi for correcting an earlier error with this write-up: a maru is a symbol, not a character.