Capture the flag mode is a mode where your main goal is to capture the enemy flag and bring back to base. Each flag is located on a little pedestal somewhere in the base. The flag is red if it belongs to the enemy and blue if it belongs to you. When your team has delivered 1, 3, 5, 10 or 15 flags before the enemy does, victory is yours. If the goal isn't reached within the time limit, the team with the most flags deliverd wins.
Use a tank with decent speed or good armor. Use the M50 Ontos, the AMX-13, Spahpanzer SPI.C. or the FV101 Scorpion because of their high speed. Use a smoke grenade as soon as you manage to get the flag to blind them and leg it. Working with your team is a good idea as well. You may go with multiple tanks to the capture point, to kill the enemy team and secure the flag.
All current maps support capture the flag mode and have them in various locations.