Affiliate Marketing, what is it and can it really produce the income several website out there promise?

I’m looking to make a little extra income. My wife is unemployed, I have two children and a stand-still job. If this method can bring me 0 – 0 more per month I would consider investing in affiliate marketing. If the majority of these claims are scams, please let me know or if you know how to produce the results as advertised please send me a link with a guide or a recommended site to read. Thanks in advance!!! One last thing, if this method is completely false please enlighten me with something else that may help. Thanks again!