In a nutshell-
a track is a track in PT, numbered 1-48
a channel is a midi channel (16 to choose from, 1-16)
a port is a place the midi gets sent to

The port concept is what gets most people.
You can have any midi channel(s) on any track. For instance track 1 in PT could be a track for the drums containing information on midi channel 10...or it could be a piano on channel 1... or a bass on channel 2. *Note that a channel can be assigned to any instrument; these are examples (although drums are almost always channel 10)

So a midi channel is just a way to have 16 different choices for midi information.
The track designation has no bearing on midi except to organize things, and have control over individual midi channels or groups of midi channels.

Now the port-
Every midi sound source you use can have the ability to use those 16 midi channels. So what if you want the piano sound from synth#1, but the drums from synth#2 ?
You *could * send the information to synth1 and thru that synth and on to synth2... but if you had a bunch of available soundsources this would not be practical to send all the information thru all of the synths and then tell each one which channel to obey and which ones to ignore, plus after 'X' amount of daisychaining the signal would start degrading... so in come the ports.

You can have a bunch of different midi in and midi out ports on a system.
Example- output port 1 may go to synth1, and output 2 could go to synth2, and output port three could go to whatever ... etc.
By assigning WHERE the signal on a given track gets sent we can keep things organized, PLUS we can send midi channel 1 from track number whatever out port 1, and then be able to use midi channel 1 again on a different track and send it out a different port, thus allowing us to get past the limit of 16 sounds from 16 midi channels.

So a midi track contains the midi information which can be on any given channel(s) 1-16.
Then that track gets assigned to a port so that the midi information on that track can be directed to whatever synth (or other midi controlled device) it is we want to send it to.

In PT we have 16 available midi output ports (1-16) and 16 available softsynth ports (17-32). In theory this is 16 X 32 available ways to get the midi information to the receiving device. Thats a lot of flexibility! The limit is usually in how many physical midi connections we have to use for the first 16 ports (1-16).
Example- an SBLive soundcard will have a midi port on it usually, as will a lot of the M-Audio cards, and many others... plus we could get a USB-to-MIDI device... these would be considered midi output devices available for use by the first 16 MIDI ports.
To use the other 16 sofsynth ports its simply a matter of right-clicking a track and selecting any available softsynth port (17-32) and then assigning a softsynth to it.

HTH and as I look back at it I see it really wasn't 'in a nutshell'... but hopefully you can fumble through this.

I do not work here, but the benefits are still awesome
Make your sound your own!