Electrical cover plates are put over electrical outlets to protect either the outlet itself or from someone accidentally damaging themselves by touching within (usually babies).
There are lots of different types of plates on offer as I'm sure you can imagine and most of these are very simple plug in designs. These work by simply being designed to imitate a plug but which is not actually connected to or designed to power anything. They slot into the outlet and stop any children from putting their fingers inside.
You can also buy larger ones that slot in and are designed to protect the outlet itself. These are usually for outdoor electrical outlets and are meant to protect the vulnerable inner components against the weather. Having an outdoor outlet can mean having to buy certain protection capabilities for it. This means buying something which will protect it against the elements such as wind, rain and snow – or any combination of the above.
Usually for the outdoor the electrical cover plates have to be more secure than to be simple plug ins. This usually means a cabinet or box which is roughly the same size as what it's protecting. This is attached either by glue or by screws over the socket and allows access and protection. Accessing the sock is as simple as lifting or moving around the front piece of the box. Protection is a simple matter of keeping the relevant protective pieces in case, these types of devices are usually made out of a clear plastic or sometimes metal.