Loosely associated with any participant, operator, or any person who interacts with tools. In computers, a user interacts with a computer component such as software (a program), a piece of hardware, or even the internet.
In this sense, a user is the person who uses a computer, who uses software, who writes email, who visits websites. A user is everyone. Of course, the term user could be used in a more real-world sense. From the perspective of a product-producer, a user would be a person who purchases and uses the product.