As i understand it, a magnet URL is a way of transferring an infohash as a URL scheme, so that it works with other programs. For example, the following magnet URL:
contains first the protocol (magnet:) which tells the OS that this is a magnet URL so give it to some app that can handle that. Since it's in URL form it's all broken down into & separated key/value pairs, so next part:
is the infohash (actual infohash is e666231c9a34be278f6cfc390099e4084b30e02, the xt is just the key for that value, urn:btih probably means something, google it).
The other ones are pretty self-explanatory, the name of the torrent (presumably so the client can display something pretty before it has downloaded the metadata) and some trackers
It's a Uniform Resource Name (hence "urn"), btih (BitTorrent Info Hash) is the namespace identifier and the rest is the namespace-specific string, the actual infohash.
