Based on the standard pitch-fork used for lifting hay, the Military fork was a two-pronged stabbing weapon with sharp tines. These were normally just points, though some of them had blades on the inside, or more rarely, the outside as well.
As a general Stabbing pole-arm, it was used in large formations of men; As it only took a short amount of room to use, it could also be used with effect by small groups, or even single men like the spear and trident. Those with Bladed tines, however, took more room to swing with, and are generally less efficient- and even more rarely used.
Mainly used by, say, peasants who are being funded by a minor baron or other land-owner who's not rich enough to use "proper" pole-arms, they are cheap and easy to make- just sharpen a pitch-fork. Because most farmers and peasants would own a pitch-fork, it was easier to make than a spear! (that is, a decent spear with a head, not just a sharp bit of wood.)
The Military fork could be used in any way a spear could, with the added ability to disarm weapons that got caught atween the tines; this could be used against other pole-arms, twisting them sharply, as well as slashing weapons like swords.
A useful offensive weapon due to its quick stabbing effectiveness, and a good weapon in defence, the Military Fork was one of the cheapest pole-arms... and, monetarily, one of the most efficient. However, in its effectivity, it is surpassed by the halberd, glaive-guisarme and the pike.
An interesting side note is it was less used than the more expensive and generally worse fauchard. This is probably due to style- which looks cooler; A big blade on the end of a stick... or a stick with two little pointy bits?