Before your site/forum appears in yahoo or google search (or any other search engine), they have to "crawl" your site.
Google and other search engines (I'll call them SE's from now) have "spiders" that do this crawling.
This are automated programs that visit websites and index their content.

But before a spider can crawl your site they have to find it.
The can find it if they visit other pages that have links to your site.
Or you can give your url directly to them.
First method is better though.

Here is the link to submit your site manually to google:
http://www.google.com/addurl/?hl=en&continue=/addurl

And the more links other website's have to your site to higher you will apear in those search results.

Haha, thanks
If you've got any questions about it, don't be afraid to ask them

You can also submit to this search engine: http://www.webwobot.com/submit.php

Yeah, been a while. And I was very busy with school (tasks & examsn)

Good post!
Sums it up nicely.
Rep added

Sure for the big SE's that can afford to run numerous crawlers from multiple data centers around the world. Does take a bit of time to spider the whole web though... not to say the entire web gets spidered. But yes, one submission does at least ensure that you're in the system. So yes, no harm can come from it.

one tip, you can submit to search engines manually, but dont over due it, can result in ban, i let search engines naturally crawl and index when they want, worked for me so far

no if you do it too much, like lets say they have a hidden limit of 20, you do it 21 times ur in trouble, because their servers are spammed with the same site, like owning a directory -- imagine you get the same link 50 times in a row in your list, are you gonna accept it 50 times? or ban that idiot?