Some cookies have expiration dates. When a cookie expires, your browser will simply erase it from your hard drive. Depending on where it came from, a cookie may expire tomorrow, next week, or the year 2010. A cookie that doesn't contain an expiration date will only last as long as your browser stays open. When you close your browser, all session cookies silently disappear.
Cookies are generally harmless.
Cookies are very popular these days. Some sites will set several cookies on each page. It would be really annoying to confirm each and every cookie you receive. Many sites won't work properly without cookies. If you are trying to clean out unwanted and unneeded files and want to delete cookies, stop and think about each cookie before deleting it.