What is actually a VPN and who should use it?

With a Virtual Private Network (VPN), you create an encrypted tunnel between your computer and a remote server. This means that you:

Hide your internet activity from your ISP (and anyone else who’s watching)
Hide your “real” IP address (your IP on the internet appears to be that of the VPN server instead).

VPNs are therefore good for:

Evading blanket government surveillance
Watching geo-restricted streaming services such as Hulu and BBC iPlayer from abroad
Protecting your Wi-Fi connection when using public hotspots
Evading censorship (both government or at school, and so on)
Protecting you when P2P downloading.

While the encryption does have some data overhead to it with typical use you won’t experience any...

It adds a new network interface which you are working with as if you are working with direct...