An idiom is a phrase where a combination of words together have a meaning that is different from the dictionary definition of the individual words. This makes idioms especially tricky for ESL students. There are over 8,000 idioms in the English language.
Check out a bit about Idioms:
http://www.rong-chang.com/idioms.htm
Especially these videos:
http://www.22frames.com/idiomlist.aspx
