Really dumb questions but I don't know the technical terms...

I would like to set up additional sites on my server but I'm not really sure how to go about doing this. I know that there is something called virtual hosting which I would like to know how to do sometime in the near future and also something else called nested domains (someplace.somedomain.anotherdomain.com) but, again, that's not what I need right now. I would like to know how or at least what it's called when you want to point your DNS server to a different site within the same domain, like this:

Instead of www.somedomain.com it would be mail.somedomain.com (a link to a wer-based mail server running on the same domain) or docs.somedomain.com (documentation for a project you're working on). This way I wouldn't need links off my main page, I could just enter the prefix to the domain and it would take me there immediately. Would this be DNS configuration or just making some changes in the web server?

Any info would be greatly appreciated. Thanks!

Well it's partly dns and partly webserver related. You got to make sure those custom subdomains (docs and mail etc) point to your webserver's IP address. Then you need to configure the webserver to access a directory in the main website directory to retrieve the documents. Something like this:

www.domain.com => /var/www/domain.com
www.domain.com/docs => /var/www/domain.com/docs
docs.domain.com => /var/www/domain.com/docs

As you can see, the same directory is accessed by 2 different URLs. In apache multiple websites on one server is called "virtual hosts". In IIS, you would go for multiple websites via "Host Headers".