A license is a contractual agreement that permits usage of copyright work in exchange for something. This can be payment of a fee, but may be anything of 'valuable consideration'. A byline or link to your site, or entry into a competition may be offered, and that is enough to form a legal contract if you accept it.

License contracts may be terribly simple, but with copyright they are often made very complex by one or both parties stipulating terms and conditions that define what will be agreed.

Never agree to contract terms unless you are able and willing to honour them. A license is a legally binding document.