A standard telephone keypad, with four rows of three keys each. The top row consists of 1, 2, and 3, with keys continuing down in order until the last row, which is usually *, 0, and #.
Such a keypad can be also used for text entry. Each key typically has 3-4 letters on it. Predictive text entry is usually used to speed this process.
Phones designed for heavy text entry may include a more text-oriented keyboard in place of a 12-key layout. Alternatives include 20-key and QWERTY.