Very quick answer, for more details see links below or google or a book about gps…
Track: A group of trackpoints (usually recorded with a gps device). Tracks are limited to 500 trackpoints in the HCx.
Waypoint: A single point of interest (e.g. summit, intersection…)
Route: A group of waypoints entered into the GPS receiver in the sequence you desire to navigate(!) them. Routes are limited to 250 waypoints in the HCx.

A .gpx file can contain several tracks, waypoints and routes.
Tracks and Routes can be interconverted e.g. with gpsbabel.

http://www.gpsmap.net/DefiningPoints.html
http://cycleseven.org/gps-waypoints-routes-and-tracks-the-difference