An idiom is a commonly used expression whose meaning does not relate to the literal meaning of its words. For example, when a person says, "I'm going to hit the books" they really mean that they are going to study really hard.
Below are some common idioms you might hear at school:
A tongue twister is a sequence of words and sounds that is alliterative and very difficult to pronounce quickly. Public speakers, such as actors, sometimes practice saying tongue twisters before a performance in order to improve pronunciation
For tongue twisters that address specific pronunciation issues, check out TesolGames' page