Secret characters are more common in fighting games, which are fundamentally based around a set of different characters. However, they can occur in many game genres.
In some cases, secret characters are given as a reward for completing a game (a process usually described as unlocking a secret character). The Tekken games are one example of this, as they actually provide empty spaces on the Character Select screen for characters that have not yet been unlocked.
Other secret characters, however, are far more secretive, and can be classified as a kind of Easter egg. Some may require the player to accomplish impressive or unusual gameplay feats, and some can only be obtained by using cheats or hacks.
Popular secret characters sometimes return as main characters in sequels of the original game. Ayane was a secret character in the original Dead or Alive but is a main character in every subsequent installment of the series.
Sometimes certain characters are not completely secret, but appear only as non-player characters at first, such as bosses or enemies, but then become available to choose as playable characters when the specific tasks or cheats are performed. They are not secret as enemy characters, but secret as player characters.
