Definately check out the references above, for a more thorough and accurate answer, but for a quick answer to your question:
Sources, Ports
One Source is like one MIDI IN on your computer, to record midi. Likewise, one Port is one MIDI OUT. The sound blaster has a couple "virtual" sources and ports ... for example, you can play directly through the sound font device, or you can play indirectly through the microsoft midi mapper (useless with the sblive, imho

). For input source, you can grab the MIDI IN, if you were going to record from a external keyboard for example, or you could use the virtual piano if you like.
If you had multiple sound cards with MIDI support, then you'd have multiple MIDI outs and ins, and therefore ports and sources. So it's like audio ports and sources in that sense -- outs are ports and ins are sources.
Each MIDI source or port can have up to 16 channels of midi data, which brings us to:
Channels
each "channel" of midi data is an independant stream of notes. somewhat like a "track" -- though in most programs you can have multiple tracks on the same channel. They just have to share the same channel properties (these properties can change over time) -- vibrato, volume, pitch bend, for example, and bank and patch .... which brings us to:
Banks, Patches
A "patch" is an instrument. For example, piano, or drums. "Banks" are groups of Patches. For example, you might have the basic general MIDI bank as bank 0, and a cool sound font library as bank 1. patch 0 on bank 0 would be piano, and patch 0 on bank 1 would be, i don't know, some weird indian instrument or something.
One important thing to know is that channel 10 on most midi setups plays through the "percussion" patch, regardless of the patch set on its channel ... the percussion patch usually has a different drum sound per note and isn't available through the normal bank/patch selection (it's a special patch that only shows up on channel 10)
there's some other things, like Controllers, which control properties such as pitch bend and vibrato, but i'm sure you'll find more info on this stuff elsewhere.
I'm sure there's some inaccuracies here but I hope this gives you a starting point

Also check out the MIDI Mania forum.