There are lots of ways to communicate online, the most common being via email and instant messaging. People also communicate via social networking sites, such as myspace, which allows you to build up a large group of friends by adding people to your friends list, and they can do the same with you.
Chat rooms are another popular way to communicate online, along with message groups (such as Yahoo groups), and forums. Forums allow you to have ongoing conversations with other people, though they are often moderated, meaning that you have to adhere to the moderators rules.
The programme SKYPE is a voice-over programme, and allows people to communicate with each other either by speaking into the speaker on their PC (or MAC), or by sending messages to each other.
Web cams allow to people to communicate online, and also allow people to see each other as though they are speaking face-to-face.