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.
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.
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.
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.