There are 3 different types of locks on a garage door.
Use the opener as a lock
The first and most common way to lock a garage door is the opener unit. When the opener and the door is in the down position the door cannot be opened manually.
T-handle lock
The second type of lock is a T-handle lock. These locks use a T-handle on the outside of the door and need a key to unlock. When you turn the handle it pushes or pulls a bar thru the track of the door locking it down or releasing it to open door. The other type of lock is called a slide lock. This lock is a small metal box with a slide handle that is located inside on the left or right side of the door.
t-handle lock for garage doors
slide lock
A slide lock just slides a bar thru the track to lock it down. At the end of the bar is a hole so you can put a padlock on it if needed. Slide locks are the most simple design but also the most effective when used with a padlock. The door cannot be opened from the inside or outside of door without a key or combination. All the other locking systems can be easily unlocked from the inside of the garage with or without keys.
slide-lock-for-garage-door
Warning, if you have an opener installed in your garage you have to always make sure the door is not locked when you operate the opener. This is a common situation and it happens all the time. The result is damage to the top section of the garage door. When I install garage door openers and the door has a manual locking mechanism I always ask the homeowner if they would like me to disable it to prevent any damage to the door.