The Internet is a bunch of tubes connected together. There can be many internets but there is only one Internet (with a capital I) that is official.
Common practice regarding the usage of the Internet
Most commonly you'll use a Browser to "surf" the world wide web, or use an MUA to send Email. If you like to chat with others, you may try an IM client if you know which protocol your friends use, or the infamous Internet Relay Chat.