I'd say the general distinction between zerg, terran and protoss in Starcraft is that zerg are biology based, protoss are psionics based, and terrans are technology based. Terrans strictly have a lower technology base than the Protoss, but they seem to be better at coming up with ingenius ways to make full use of what they have, while protoss technology largely seems to be based around augmenting or reproducing the effects of their psionics.
I'm far from a competitive player, but the distinctions in strategy can probably be summed up as the following (keeping in mind that Starcraft gives everyone flying units, and that the building strategies don't easily convert to the typical 4X):
The Zerg are the Swarm. Whether on the ground (zerglings) or in the air (mutalisks) the zerg are basically built around swarming the enemy with massive amounts of relatively basic units. They have specialised units higher up the tech tree for dealing with situations that the basic units can't handle on their own, but basically, they swarm the enemy. In Master of Magic terms, a good analogy would be gnolls, barbarians or some other race that can crank out hordes of decent troops early on, coupled with use of summons to crack things that sheer numbers can't.
Terrans are the closest to a modern military force. Numerically they're between the zerg and protoss, but their main advantage is the long range of their units (and the corresponding ability to concentrate fire and do unto others what they would do unto you first and at longer range). In army structure and tactics, the terrans can be compared in Master of Magic to elves (especially dark), halflings, or nomads.
Protoss are about small numbers of powerful units (relative to their position on the tech tree) and potent special abilities. Beastmen, dwarves and trolls might all be good analogies, as long as they're backed by a potent wizard that's not afraid to cut loose with spells. (High Men DON'T count, since although they have arguably the best non-summoned unit in the game, they have to do a lot of tech-tree climbing to get to it, and until they reach the top of their tech tree they're pretty baseline. That's more the equivalent of the playstyle of gambling to get to the top of the tech tree with minimum other investments... which I guess is something protoss players have historically been more prone to.)
