I'm not Canadian, but generally, boulevards are wider than streets and avenues and accommodate more through-traffic (have multiple lanes and has a divider/median). In New York (City), avenues run east-west, streets run north-south. The distance between avenues is longer than the the distance between streets.

As tourusan says, there are no official definitions. Sometimes it is just marketing. But here are the general nuances of the different words. In size, from smallest (least lanes, least traffic) to largest => street, avenue, boulevard, drive. In Chicago, a boulevard often has a strip of grass or park in the center and/or on the sides, and is more often tree-lined. A drive is normally a more scenic road and often has more limited access (eg has on and off ramps). Again, there are no official standards, though. So if you are building a subdivision, having an "avenue" or "drive" may sound much better, even if it's not all that wide or scenic.