It's a protocol used to setup a vpn tunel, the basis of all VPN services. It's how computer connects to the network, how it negotiates the encryption the tunel uses and what is being sent and rwceived through it.

Other examples are: mentioned wireguard, ipsec and SSL. There's more, but more obscure ones too.

Just the same as other VPNs, just different protocol.

It's a regular point to point VPN just like wireguard and ipsec. Based on openssl. So you have a client and it connects to a single server. You can also connect a network to another network but usually you use a dedicated router for it. Only if you connect individual clients would you use an app.

There's other VPNs these days which are substantially different, called Mesh or Overlay VPN. These are ones like tailscale and zerotier. They are different in usage because each client can talk together independently. This means even on a shared network each client will have the VPN app. It's used more for personal networks, not really private anonymous access. For those you explicitly don't want to talk to other clients so the usecase makes for different tech. For this reason anonymous VPN providers never use mesh tech.

I use both myself. OpenVPN for torrents etc. And tailscale for connecting to my home stuff from my phone and laptop.

But OpenVPN is a very classical VPN type.