Once you start playing, a square may look like any of the following:
Gray square: A normal square that has not been uncovered. When you start the game, all of the squares are like this.
White square with number: A square that has been uncovered. The number inside the square indicates how many mines surround that square.
White square without number: A square that has been uncovered but does not have any mines surrounding it. When one of these is uncovered, the surrounding squares are also uncovered automatically, since they can't contain mines.
Red square: A square that you think contains a mine.
Yellow square: A square that you think might contain a mine. When you use this is up to you; yellow squares are treated exactly like gray squares by the computer.
Red square with explosion symbol: A square that has been uncovered that contains a mine. If mistakes are allowed, this is similar to a red square; otherwise, it just shows you which square you clicked when you lost.
Red X: A square that contains a mine. These appear once the game is over so you know where all the mines are. These also appear if you uncover all the squares before marking all the mines.
Gray X: A squrare that does not contain a mine. These appear after you lose on squares that you marked as containing mines, even though they didn't actually.