(Augustus, alt.html, 2002.02.23) --
There are pretty much 2 kinds of ads you can go for... a general banner rotation service (you put their code on your page and they supply ads to it) or looking for specific ads that target your target audience.
In the first case, most of these services have a restriction that their ad must be viewed atleast 3,000+ times a month (dependant on service) and generally pay about $1 per 1,000 banner impressions.
In the second case, this is where you find people to advertise on your site whose target market is your audience... example: If you are running an online job club for your city, these would be job boards, recruiting agencies, other job clubs, community resources, etc, that service the area your city is in... for these, you can get anywhere from $1-15 per 1,000 banner impressions (sometimes more, but the whole advertising market is going downhill)
You might look at the first one and think "Hmm... well, my site sees about 50,000 a visitors a month... thats $50 a month I could make... if I sign 10 of these services, I could make $500 a month..."
Unfortunately, it usually doesn't work that way... a lot of these agencies have restrictions on their ads nowadays... namely 2 big ones: (1) You can have only so many ads on your page. and (2) Their ads must appear within a certain area of the page (and suprise suprise, they all seem to pick the exact same area... within 2 or 3 inches of the upper right corner of the site)
Now, that's if you want to go for the "set" income per month from these guys... you can also look for affiliate programs out there... ie: casinos, credit cards, etc... these don't pay you a set fee, but will pay you a commision on how much money they make off you. (ie: You get $50 each time somebody signs up and is approved for a credit card, or $1 each time somebody submits an email address to your mailing list, etc)