^^ What Shufti said. Flickr has a lot of uses depending on how you want to use it:
At its most basic its an image host which lets you put your photos up onto the internet. They allow some other media, but the dominant content must be photography and in addition it must be your own or that to which you have a copyright licence to post online.
Flickr also takes part in the Creative Commons licences which are designed to allow people to share their work (be it art of any kind) whilst allowing others to use it in a limited manner legally speaking. The various licences are described on flickr, but if you want total control you can always set it to reserving all rights which means any use requires your permission. You always preserve your copyright on the work legally speaking - this won't stop a theft, but means that you can take legal action on any thefts. It should also be noted that of the major image hosts (including Photobucket and Facebook) flickr is the only one that does not have any legal trickery with getting rights to photos that you upload to them. Photobucket (for example) has a right to use any uploaded content for any purpose (including making money and selling them to 3rd parties) so long as the images remain on photobucket in an open account (closed private accounts are still protected).
In addition flickr has a range of communities within itself - rather like forums. A great many of these are award groups mostly aimed at giving glossy shiny things in comments and such. You can take part in this or not (your choice) and there are a great many good informative groups if you search for them. From ones like the strobist group which deals with lighting - to the canon and nikon equipment groups - through to art and critique groups.
Flickr is free by default, though organisation structure and photo access are limited in this mode (you can only see your latest 200 photos - earlier ones are still saved, but not viewable in flickr - if linked to from other sites the links remain working. In addition you can't see fullsized uploads if you upload them). An annual subscription is less than a magazine subscription and if you make good use of the groups the info you can tap into along with the hosting services more than cover the cost for membership.