A virtual private network (VPN) allows your computer (client) to join a remote network. Communication between your computer and the VPN server is encrypted and secure.
An example of this is your company laptop having to 'sign in' using a VPN client before being able to access the company servers. An network traffic between you and the company VPN server is encrypted.
Anonymity is another reason to use a VPN. There are commercial VPN providers that allow you to join their VPN, and surf the internet from their network. Instead of requests originating from your home external IP, once you connect to the VPN, all internet requests will appear to originate from the VPN's external IP.
When looking for a VPN provider these factors were important to me.
Anonymity is important.
A cap on the transfer upload and download may be applied.
There should be multiple servers to pick from.
A standard VPN client without the need for any VPN provider specific software to be installed.
This is required for some applications.