Back-end developers use PHP, Ruby, Python, and other server-side languages to create applications and connect them to a database on one end and the front-end of the website (so people can use it) on the other. Back-end developers are less concerned with design and more focused on security and making sure all of the moving pieces are working together smoothly behind the scenes.
Full-stack developers do both front- and back-end development. It isn’t unusual for a web developer (particularly a freelancer) to have skills in both types of languages. For instance, WordPress development is primarily front-end development that dips into back-end development for customizing templates and managing databases. However, a full stack developer isn’t someone who just knows a little bit from each column; she can build a site from start to finish on her own.
Some developers focus on an actual technology rather than a specific type of work or end product. It’s possible to be a PHP developer, for instance, if you have gained an impressive proficiency in that one language. In that case, you can be hired to create the PHP for any app or website that seems like a good fit for you.