The Linux kernel itself treats users are mere numbers. Each user is identified by a unique integer, the user id or uid, because numbers are faster and easier for a computer to process than textual names. A separate database outside the kernel assigns a textual name, the username, to each user id. The database contains additional information as well.
To create a user, you need to add information about the user to the user database, and create a home directory for him. It may also be necessary to educate the user, and set up a suitable initial environment for him.
Most Linux distributions come with a program for creating accounts. There are several such programs available. Two command line alternatives are adduser and useradd ; there may be a GUI tool as well. Whatever the program, the result is that there is little if any manual work to be done. Even if the details are many and intricate, these programs make everything seem trivial. However, section 9.2.4 describes how to do it by hand.