High Street runs north and south into residential areas, while Main Street runs east and west into business districts. Periodically, you'll come across items that, when you take them will confer certain abilities or powers on you. If you type help and enter, you'll get a list of files you can read to learn more about the MUD's commands. Another way to communicate with somebody regardless of where on the world they are is through your pager. Other MUDs and MUCKs may have different commands, but generally use the same basic idea of letting you navigate through relatively simple English commands.
MUDs can prove highly addicting.
The important thing to remember is that it's the fantasy world of possibly hundreds of people, and not just yours in particular. Always remember that you may meet these other people some day, and they may break your nose. People who treat others badly gradually build up bad reputations and eventually receive the NO FUN Stamp of Disapproval.