The ‘only’ user on Ubuntu LAMP is root user which you access by default.
They other users, which are here to support running services, but you can’t login to them or use them beside service for which they are made.
One example of this user is www-data which is here for web servers.

To list all users, you can use command:

cat /etc/passwd

Example output
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
...

It shows user and shell for user (what they see once they log in).

You can create your own user and you’re recommended to create a new non-root user for managing server. Steps for doing so are covered in Initial Server Set Up - Ubuntu 14.04.