There are three different methods and it is essential to use the correct and appropriate method in order to avoid unintended consequences.

Permission is a simple, informal agreement that the other party may make use of your work. Really, this means just saying 'yes' to their proposed use. You would be wise to specify exactly where and for how long and in what context they may use it, but if you start asking for payment or a byline in return you are creating a license instead. Informal permission stops short of forming an enforceable contract, so you have no real redress against misuse except copyright law, but conversely the other party can impose no conditions or penalties. Permission is really only appropriate for non-business use by friends and family.

A license is the usual form of agreement, and comprises a legally enforceable contract. A contract is an exchange of value between two parties, and imposes terms and conditions on both which specify the nature of the contract. It is essential that you read the terms offered by the other party and are careful about specifying your own. The small print of commercial publishers, organisations inviting reader submissions and competition rules must be read carefully as they often include copyright grabs and impose serious legal liabilities. Licenses are subject to contract law, and damages my be sought by either side for failure to complete the contract.

Assignment of copyright permanently and irrevocably surrenders all copyright and transfers it to another party. Assignments must be in writing, but are sometimes buried in contract terms and conditions or simple forms, so be very careful. Once you have assigned copyright you cannot use the photo you made without permission of the copyright holder.