A double negative is usually produced by combining the negative form of verb (e.g., cannot, did not, have not) with a negative pronoun (e.g., nothing, nobody), a negative adverb (e.g., never, hardly) or a negative conjunction (e.g., neither/nor).
I didn't see nothing.
I did not have neither her address nor her phone number.
It wasn't uninteresting
She is not unattractive.
A double negative gives the sentence a positive sense. (e.g., "I didn't see nothing" is similar in meaning to "I saw something.") A double negative is not always an error. The latter two examples, meaning "It was interesting." and "She is attractive.", are fine.
I cannot say that I do not disagree with you. (This brilliant quote by Groucho Marx can be considered a triple negative. If you follow it through logically, you'll find it means "I disagree with you".)
Double negative with neither/nor
Glossary of grammatical terms
Click on the one which is NOT a double negative: