Roughly each month, a new patch is released. The first place this patch will be is www.thothie.com/ms but the file is normally uploaded to several other sites, such as ModDB. These patches contain new items, spells, monsters, scripted events, and FuzNet map compatibility additions, amongst other various things.
In online mode, many servers connect to a central server named FuzNet. To gain access to FuzNet, simply connect to a server which has the password - or contact Blasto to get the password for your server to connect to FuzNet. You will need the latest patch installed to have access to FuzNet, the official MS:C central server.