MSN is a number of things. It was and I think still maybe an ISP but its also a website. The website has general news features etc etc and of course Hotmail which is microsofts free e-mail service. Not that I have any problem with them but its essentially a propergander tool for Microsoft.
BadBob is correct. MSN stands for Microsoft Network - it is just an abreiviated way of describing a range of services offered by Microsoft over the internet. Things like news, e-mail, search engine, user groups, chat...etc...etc.
Other examples of this type of service are Yahoo or Lycos.
Both BadBob and rinkytink are absolutely correct. When some people talk of MSN, they just mean the chat programme, known as MSN Messenger, which you can use with any ISP (as long as you haven't got excessive firewalls in place) and can be downloaded here: http://messenger.msn.co.uk/Default.aspx