Negative numbers

Here's a great introduction from Kahn Academy.  It's a video explaining the nature of negative numbers.

http://www.khanacademy.org/math/arithmetic/absolute-value/adding_subtracting_negatives/v/negative-numbers-introduction

here's one for adding negative numbers

http://www.khanacademy.org/math/arithmetic/absolute-value/adding_subtracting_negatives/v/adding-negative-numbers

Adding numbers with different signs

http://www.khanacademy.org/math/arithmetic/absolute-value/adding_subtracting_negatives/v/adding-integers-with-different-signs