The 1st one is the user. The 2nd one is the group.
Linux knows several layers that you can set permissions to that would apply to the user. One of these layers is a group: you can create a group, set permissions for that group and then add users to that group. Changing the group applies it to all the users to that group.
If you have 100 users it would be rather time consuming to set permissions to all of those users seperatly if large amounts of those users will have the same set of permissions.
Have a look at AddUsersHowto on how to set this (bottom part includes command line options).
When you type
groups in command line you will see the groups you belong to yourself. I myself belong to the groups
rinzwind adm cdrom sudo dip plugdev lpadmin sambashare jupiter. For most of these groups you can check this page for some information about them but there purpose should be fairly obvious.
jupiter is software that is for CPU management that I added after install; the rest should be default.
dip is the odd one out in naming but that is for modem access.
lpadmin for printers etc.