Like its brother, the axe, the pick-axe was an early used weapon and tool- and one of the most useful innovations in history. Whereas most weapons have been modified or changed- axes evolving smaller heads and longer handles, some becoming swords or daggers being balanced- the pick-axe still exists as it always has: A sharp prong on top of a good shaft.
Whereas the axe begot the sword, the pick-axe has created no new weapons (save possibly the obligatory polearms, like the warhammer, lucern hammer and halberd) because of its wonderful simplicity. It has had no need to be modified.
Evolved like its brother from the dagger and hammer, instead of using the edge, the pick-axe uses the point of the axe to deal damage, resembling the dagger more than the hammer. Superior to the hammer from the point and the dagger from its weight, the pick-axe was one of history's most effective- and underrated-weapons due to the horific damage caused by it if it penetrated the flesh.
Again, it was a useful tool; while the axe was used to cut things up or break things down, such as wood and trees, the pick broke things up and cut things down, such as rocks people. Where the axe cut trees to gather wood for building and making fire, the pickaxe dug the earth to plant crops and break slate for one's rooftop.
Due to the stabbing end of a pick-axe, the internal wounds it deals a creature are deep and blood well; they would take much time to heal and would normally scar if the beast survived much longer.
As metal came into use, the pick-axe played less of a role than the axe, due mainly to the fact that an axe took less effort to use; once you hit with a pick, you also have the difficulty of pulling the head out of the reciever.
Double-headed picks evolved mainly for mining, and were quite effective; the additional weight from the pick allowed it to strike deeper into the rock, but the second pick also meant that you had to be careful when swinging it around. The last thing you want to do is accidently kill your best miner. It's probably also the last thing he wants too, actually.
One of the reasons it was a less popular weapon-though horribly effective when it hits- was that they were normally overweighted. The fact that one could swing it to impale a target was more than enough damage ability, the added weight that was normall added tended to be overkill, dead weight stopping one from using the pick effectively, with speed or skill, and as the damage was so high it increased the chance that the pick would be stuck or even break!
While a pick is a weapon for a skirmisher due to the large amount of room needed, it was used by many mercenary footmen; its presence of the battlefield never caused a general to start rethinking his plans (unless he suspected they were hiding pikes), and being confronted by a pick-wielding warrior was no worse than meeting a sword-wielder. Few tactics that used a pick were developed, and the pick wasn't a weapon in every general's army "just in case they have an elephant".
With good resources and knowledge, a pick and an axe, one could happily build a house and live in the wilderness.