System vs Regular Users

Linux users can be "system" users or "normal" users. 

• Regular users typically have UID's at 1000 or higher and are allowed to log into the server.
• System users often have a user id (UID) below 1000 and cannot be used to login. They are usually used to run programs. For example, users www-data or apache are often used to run web servers.

See the file /etc/login.defs to find variables like UID_MIN and UID_MAX set for system vs regular users:

#
# Min/max values for automatic uid selection in useradd
#
UID_MIN                  1000
UID_MAX                 60000
# System accounts
SYS_UID_MIN               201
SYS_UID_MAX               999

#
# Min/max values for automatic gid selection in groupadd
#
GID_MIN                  1000
GID_MAX                 60000
# System accounts
SYS_GID_MIN               201
SYS_GID_MAX               999