Software products never consist of one file and do it all at once, they are divided into parts. Some part deals with the conclusion of data collection which the user can enter text in forms, downloads, clicks and movement of mouse pointer, sound from a microphone, a camera image. This is called the front-end (front-end). Then it is that part of the software system which is able to save to the database, modified, transferred to another program. This invisible part of the software system called backend (back-end).
Once for the whole site could be answered by a single person. He page in Dreamweaver, sketch, and texts with pictures on the website will upload and hosting will do. And now in web technology so large a variety that one person has become very difficult to know everything. Therefore, there were the developers for front-end developers and back-end. Generalists who know both parts, called "harvesters" or "full-stack developer".