- ○　まる
  - **OK; correct; yes; supported; available** (like <code>&#10004;</code>; note that the check mark usually means "wrong" in Japanese examinations)
  - masked/censored character (like `*` in English used to mask characters in certain words; see [this][1])
  - rival horse; second likely to win (horse race; favorite horse is marked with `◎`)
  - win; victory; [白星][2] (when used as opposed to 黒星 = `●` = lose)
- ×　ばつ、ぺけ、ばってん
  - **NG; wrong; no; unsupported; unavailable**
  - masked/censored character (mainly for sexual reasons)
  - multiplication (math)
  - lose (game)
  - dead; extinct; obsolete (word, species, person; like <code>&dagger;</code> in English)
  - used to denote romantic relationships, especially in *yaoi* fandom (like "slash" `/` in English)
  - finished (calendar, event)
- △　さんかく
  - **partially OK; partially supported; nearly sold out; between ○ and ×**
  - second move; [後手][3] (in shogi, when used as opposed to `▲` = first move; sometimes inverted (`▽`))
  - negative value/earnings (finance, when positive values are marked with nothing)
  - money gain; increase (stock market, when used as opposed to `▲` = loss)
  - dark horse; third likely to win (horse race)
- □　しかく
  - unavailable character; glyph missing (see: [豆腐][4] and [*geta* symbol][5])
  - full-width space (as opposed to the dot `・` used to denote half-width (ascii) spaces)

And these symbols can be used as the casual placeholder for other arbitrary words/characters. (See [this][6])

As you can see, depending on the context, filled symbols often mean the opposite things to the unfilled ones.


  [1]: https://japanese.stackexchange.com/q/11810/5010
  [2]: http://jisho.org/search/%E7%99%BD%E6%98%9F
  [3]: http://jisho.org/word/%E5%BE%8C%E6%89%8B
  [4]: https://japanese.stackexchange.com/q/5129/5010
  [5]: https://en.wikipedia.org/wiki/Japanese_typographic_symbols#Other_special_marks
  [6]: https://japanese.stackexchange.com/q/12907/5010