Teamspeak and Ventrilo are inherently rather similar applications. Both of them use VoIP technology in a client-server layout, allowing game players and others to talk online. That means that many users install their software on their home computer, called the client, which is organized through the server, or central hub. This server should be placed in a well connected local data center to keep latency at a minimum.
Gamers using these applications are able to plan strategies, talk to one another in real time, and communicate with one another without having to take the time to type. People who don't game can also hold meetings using multiple people at once more effectively than many other methods.
Group study is a lot easier using this type of application, as is planning family get togethers, or any other occasion where you need to allow multiple people to talk in real time, but don't have conference call capabilities. It's a lot cheaper and easier than making multiple long distance calls, and it's a lot easier to organize things when you don't have to relay information. Plus, there's no per minute fee on Vent servers, and they're easy to use.