VPNs: main concepts cheat sheet

‘VPN’ has become a catch-all term nowadays.
What does VPN mean ?
→ A software used to insure privacy on the internet ?
→ A way to connect 2 distant private networks together ?
→ An encryption mechanism ?
→ A tunneling protocol ?
→ A type of router / firewall ?
→ A layer 2 / 3 / 4 protocol ?
I’ll try to shed light on everything above.