for "circle." Written 丸 in kanji. There are three different "maru" symbols in Japanese.
One is written 〇 and can alternately mean "correct" (similar to a check or tick in English) or "zero" (especially in prices, such as 一〇〇円 "one hundred yen"). This maru is also sometimes used to censor offensive words, much as asterisks are used in English.
One is written 。 and is used in the same way as an English period. Unlike the Chinese full stop, the maru is kept close to the preceding sentence, rather than equidistant from both sentences.
The last one is a diacritic added to kana characters to change "h" sounds into "p" sounds (ほ ho becomes ぽ po... thanks to shro0m for pointing that out).
Maru is also a designation for ship names: cf. Ehime Maru or Kobayashi Maru.