Types of Internet Advertising

Can you help me with the types of internet advertising that sites use.

Background:
We are looking at setting up a niche help line as most of the contributors will be voulnteers we will need to look at forms of advertising to generate income for the site.

Info required:
How do advertisements from other organisations and sites generate income for the site?
How do the different sorts of advertising work/differ
a) for the site advertised on?
b) for the advertiser?
Who (sort of advertiser) is best to approach for advertising?
Any other info you think it would be worthwhile knowing.

Thank you for your help

Sheryl