Cookies are needed if you want to stay logged in. Forums, emails, etc uses cookies. They are useful in this sense, otherwise, you have to log in every other second.

Cookies aren't viruses. They do no harm, except if they become tracking cookies.
Tracking cookies, like their name, are used to track your computer/internet usage.
If you ever been to CNET's website, you will see a lot of doubleclick.com advertisements around. This website is an example of using cookies to track you.
Most advertising websites do this. One way to prevent to them is to use a Hosts file.
A list of websites blocked by Hosts fileRead and download Hosts fileRead more about cookies
here.