A user is someone who employs or uses a particular thing, like a user of nicotine or a user of an internet site. Since you are reading this, you are a user of Vocabulary.com. Congrats!
To use something is to employ it or operate it, so a user is someone who uses or takes advantage of something. If you have a computer and use it for anything, you're a computer user. If you play a video game, you're a video game user. As you can see, this word often applies to technology. That's why they call the first part of a login the username. The people who design technology always have to keep the users in mind; it's important to make tech gadgets that are user-friendly.