With sites like Facebook and Classmates.com, you can find old friends. But you can also talk to your existing friends in many, many ways online.
IM (Instant Messaging)
With instant messaging, you install a small software program called an IM client on your computer. With this program, you find people who already use IM, and add them to your "buddy" or "friend" list. They're then displayed in a big list in your IM client. If they're listed as "online," then they're using the program at the same time you are, and you can send them a text message, directly to their computer.
There are many IM clients and services, each popular in different areas.
Skype
Skype is an internet telephone service. It works kind of like IM—you install a small software program, and add other users to your friend list. But when you double-click on them, the computer starts a voice call to the other person. If you both have microphones and speakers on your computers, you'll be able to talk.
Skype also supports webcams for video chat. You can even call regular phones, though that costs a few cents per minute. Computer-to-computer voice calls are completely free.
Forums
Web forums provide a simple way to hold discussions online. Each forum is split up into multiple boards, each with many discussions.
There are many different flavors of forums. Some allow anyone to view or post messages; some require you to create an account before you can post a message; some even require an account before you can view messages.
Twitter can be thought of halfway between IM and forums. It's a website where you create a free account, and find your friends a "follow" them (which is just like adding them to a buddy list). All your friends' messages appear on your homepage. You can post messages, which all your friends can see.
So, Twitter lets you have a long-term conversation with lots of friends at once.
IRC
IRC (Internet Relay Chat) is a free service for group chat. While IM also has group chat abilities, IRC was designed exclusively for group chat.
To chat on IRC, you first pick an IRC network, which has one or more servers. You connect to one of their servers using an IRC client (a piece of software for IRC chatting), and provide a name that you want to use. Once connected, you can view a list of the rooms, or channels, on the server, and enter any number of them and join the conversation.
There are IRC channels for any number of hobbies and interests.