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