SEO Stands for doing online marketing for your business. in short if you have business website and you post your business in any free adposting or directory site means you are doing marketing for your business it's called SEO. this is short informative defination of SEO for easy to understand.
you can get various types of SEO related jobs at freelancer market, elance and odesk. I guess you heard about these sites. But you have to be expert in SEO in order to mantain good relation with clients. Goodluck..