What is a rhyme?

Rhyme is one of the essential elements of rap.  Webster's defines the word rhyme as "a : correspondence in terminal sounds of units of composition or utterance (as two or more words or lines of verse) b : one of two or more words thus corresponding in sound c : correspondence of other than terminal word sounds."

Uh... what?

In short, rhyme is two words with the same endings.

Are there any tools to help me rhyme?

Rhymezone: Just type in a word and see a list of every word (or phrase) that rhymes with it. 

Flocabulary: This website has a list of very interesting rhyming words.  If you've got a chance, check out some of the other pages on here too-- some neat stuff.