What is the group "adm" and for what it's use? and what are the differences form the group "admin"?
adm: Group adm is used for system monitoring tasks. Members of this group can read many log files in /var/log, and can use xconsole. Historically, /var/log was /usr/adm (and later /var/adm), thus the name of the group.
admin: The admin group is used to grant sudo access on ubuntu 11.10 and earlier. It's still found on 12.04 for backwards compatibility.