Morgwen,
no I'm just that guy lurking arround in Gnutella Development without having an own client.
The difference, okay it's not very much... propably not more as from banana milk to strawberry milk (add alcohol too both, if you are not a milkdrinker). You see a similarity, but at least they have a different taste.
Multisegmented downloading: You start to download a file from a host. Then you find other hosts which offer the same file, so you start to download from them too, but from another file positions (a little bit later inside the file). As a result you parallel download multiple segments of a file. It's much faster as we know, great!
If you need a visualisation, just have a look on Flashget (a famous download manager). It shows exactly how many parts of a file it has downloaded (blue dots) and how many segments (they call it 'jets') are parallel downloaded (streams with red dots). Optional use eDonkey, AFAIK they also provide a download visualisation.
Swarmed downloading (as far as I interpret this phrase): You download one or more swarmed segments of a file, from as many hosts as you find. One single host does not have the full file, only some very small segments. Those segments are tiny compared to the segments from multisegmented downloads (described above). While with mutisegment downloading you download a few segments parallel, with swarmed downloads you need to download hundreds/thousands of segments until the file is finished. You can mix multisegmented with swarmed downloads. Before I forget, all those swarmed segments has to be swarmed first (=distributed and uploaded)... exactly this is not implemented in any Gnutella/FastTrack/eDonkey client yet. Swarming is theory, it is thought to make more use of modem users, balance load between servents and increase overall bandwith/availability.
When ppl talking about swarmed downloads, they usually mean 'multisegmented download'. I guess, 'swarmed download' is easier to pronounce.
Greets, Moak
PS: Oh, and to make the whole issue very (!) confusing, there is Bearshare wich claims to have "Multi-Source Downloading"... which is absolutely not what it looks like! Not multisegmented or swarmed downloading, it just means 'resuming' (= when one host drops, they download from another). A better phrase would be "Single-Source Downloading" here. Commercial Gnutella vendors like marketing names.