and what is important to look out for when using such services? Does it matter what protocols it uses e.g. OpenVPN, PPTP, etc?
The concept of a VPN is that you dial into a remote network and your computer acts as though it's a part of the local network (hence virtual private network). When you dial into a VPN, your local computer creates a secure tunnel with the remote network that the data is transferred over. That said, when you use your VPN, you're remote connection negotiates all the traffic and it's transmitted back to your computer via your secure tunnel. When you browse websites or do whatever it is you do online, your originating IP address will be the public IP of the VPN and not your local network.
As far as protocols are concerned, I think it's generally accepted that OpenVPN is more secure than PPTP.
StrongVPN is a VPN host. What it allows you to do is route traffic through to it, then access other resources "through it."
Meaning, as DKNUCKLES stated, professionals use it to connect into a work network. Their computers can then route traffic (say destined for a resources, such as a file server on the work network), through the VPN tunnel to a resource. The VPN tunnel makes your workstation appear sort of like a workstation on the local network (at least for certain traffic).
StrongVPN (product), Ipredetor (product), other VPN service, and even ssh tunnels (concept) are some ways to route whatever qualified traffic you wish out to other servers, via another server.
Basically, you connect to StrongVPN. If you choose to route your web browser traffic through to it (HTTP otherwise known as TCP port 80), it will appear to the web servers you are coming from StrongVPN's server(s), not your own internet address.
It's useful for people who are in countries with Internet censorship. This is really the only legitimate use case.