This is historic more than anything.
The first VPN client Cisco had was written by a company called SafeNet, we purchased it off them and rebadged it as the CiscoSecure VPN client. This came in two versions, 1.0 and then later 1.1. You could use this VPN client to connect into an IOS router and a PIX.
Later on we purchased a company called Altiga and sold their products as the VPN3000 range. Part of this was their own VPN client which we called the VPN3000 client.
A side note, we soon after purchaseda company called Compatible Systems and marketed their product as the VPN5000. This company also had their own VPN client, which still is called the VPN5000 client. You can only use the VPN5000 client to connect into a VPN5000 concentrator.
Anyway, around this time Cisco figured that we had too many VPN clients also. The decision was made to use the VPN3000 client as our main client and get rid of all others. This new client was called the Unity VPN client, or just the Cisco VPN client, and started at v3.
You can now use the Cisco VPN client to connect into an IOS router, a PIX and a VPN3000 concentrator. Because the VPN5000 has been end-of-life'd, you will not be able to use this client to connect into one of these. The CiscoSecure VPN client is still supported, but no changes or work is being done on it and it will eventually be phased out altogether.
If you're wondering which client to use, go with the Cisco VPN client, it's the only one we're currently doing any work on.
As for PPTP and L2TP, they're Microsoft protocols inbuilt in Windows, so you can also use these to connect to an IOS router, a PIX and a VPN3000 concentrator if you don't want to add additional software to your PC's.
Hope that helps.
