The IT environment today in any organization is a sort of a mixture where many different Operating Systems are used, and Windows Active Domain Controller exists in majority of such organization.
Below is a 4 step How to on "Connecting Linux (Ubuntu) Systems to Active Directory Setup"
1) Update the Software Repositories
sudo apt-get update
2) Install Required Packages (likewise-open)
sudo apt-get install likewise-open
3) Join The Domain using Domain Administrator Account, replace Administrator with Domain Administrator Username and fqdn..domain with the Fully Qualified Domain Name
sudo domainjoin-cli join fqdn.domain Administrator
4) Update the Startup Scripts
sudo update-rc.d likewise-open defaults
sudo /etc/init.d/likewise-open start
Thats it !! you have now joined the Domain, Log Off or preferably Restart the Machine.
Now you can login using Active Directory Credentials remember appending DOMAIN\ before your username
User Name to Login : DOMAIN\username
This will also create a home Directory for Domain Users under /home/DOMAIN