It is like ventrilo, a program that allows you to join groups and talk to people, "The geeks" is a pretty good group to join if you want to learn about new fun games and meet some new people to play them with.
No, Raidcall is a VOIP client which allows the user to join chatrooms and communicate via their microphones. It's comparable to Teamspeak, Mumble and Ventrilo. Raidcall is also the main sponsor of a large e-sports team, if you still had any doubts over its legitimacy.
I've never used it personally so I can't say for certain. You would obviously need a microphone to TALK, but you may be able to use it to LISTEN without having one.
Two groups that i personally like is "The geeks" and "J4KClan". . . The geeks are a public group that let anyone join, they have alot of channels that have titles of great games so you can find new games and meet new people to play them with. J4kClan has been around for a bit, sence its a clan you will have to talk to the leader or someone to get in. Both groups are good choices (personally "The geeks" for me).
Two instant messenger services are GChat and Facebook chat. On GChat, you can message back and forth with other people who are online and have gmail. With Facebook chat, you can message with anyone on your friends list who is online and available for chat.