What is the difference between idioms and proverbs?

An idiom is a phrase with a meaning that cannot be understood from the meanings of its individual words. Here are some examples of idioms:

• to be fed up with (=to be tired and annoyed with something that has been happening for too long)
• to rub someone the wrong way (=to irritate or annoy someone)
• to do something by the skin of your teeth (=to complete something, but not well. “She passed the test by the skin of her teeth” means she received a very low grade, but not low enough to fail.)

A proverb is a short and popular saying that gives advice about how people should behave or that expresses a belief that is commonly thought to be true. Here are some examples of proverbs:

• Don’t cry over spilled milk. (=Don’t get upset about something that has already happened and cannot be undone.)
• People who live in glass houses shouldn’t throw stones. (=Don’t criticize other people for their faults if you have faults too - and everyone does.)
• A stitch in time saves nine. (=Fixing a problem right away will take less time than fixing it later on.) 

Like idioms, proverbs often have a special meaning that is different from the meaning of the individual words put together, but in a different way than idioms. The literal meaning of an idiom often doesn’t make sense, and idioms can be impossible to understand unless you have learned or heard them before. 

The literal meaning of proverbs is clearer, but the intended meanings of these expressions are not the same as their literal meanings. For example, as shown above, the meaning of the proverb “Don’t cry over spilled milk” is not about spilling milk at all.