Locking a utility meter is one of the ways to keep people from tampering with it, but it presents a unique difficulty. Meter locks must be accessible to repair personnel and the utility company, which means they must be universally unlock-able but still difficult to tamper with.
Barrel locks are small metal rod-shaped locks. They have angled heads that help prevent gripping and, potentially, pulling them out, and they are difficult to break out of the lock. It can only be opened with keys issued by the company, and the keys are only issued to individuals who have been vetted and registered with the company that issues the lock, making them very safe.
Wire locks use a piece of wire that wind around a plastic core. These locks are very flexible and can be used in a variety of situations. The thing that makes them so flexible--the wire--also makes them weaker as locks because the wire can be cut.
These locks can be used instead fo a barrel lock; they have a bolt that locks into a plastic case. They must be removed with a special tool, and, because they clamp down on the meter, it's difficult to get a wire cutter or blade under the ring to pop it off or cut it. The locks are tamper evident, so it's easy to know when someone has been trying to pop it.
These are the most commonly seen utility locks. They have small plastic locks with a strong metal wire and is shaped like a padlock. They come in many colors and shapes. Because they are so common, they come in the largest variety of colors and uses. Some can be tamper evident, so they will show if someone has tried to open them.