Microsoft Network Messenger is also known as MSN, and is a product of Microsoft.  It is a program people use to communicate with other people who also use MSN Messenger in real time.
To use MSN Messenger it is necessary to have a passport account, which is linked with an email address that you would use to log in with.  All Hotmail accounts are already set up with this Passport. You can add users to your contact list by clicking on the top left hand corner where it says Contacts, then when the pop down menu appears click on Add a Contact. Then you would type in their Passport Registered email address. Once they have confirmed you to their list of contacts you can send messages to them when they are online.  If you want people to know that you are busy or away you can change your status.  This lets everyone in your contact list know that you are away from the computer or that you are busy.