A pick is that tool you see on old films of miners, with a wooden handle and a long, curved double-ended blade made from mild steel. In some cheaper tools the handle is made from fibreglass, but that can break so wood is better, and frankly, there's not a lot of difference in cost, so if you get the choice, select a wooden handle.
Tem42 says, where I live (rural NC, USA) the wooden handle is actually cheaper.
The handle is about 4 feet or a metre and a half long, while the blade is about the same length, extending half that distance either side of the handle. One end of the blade of the pick is fashioned into a point. The other is formed into a narrow blade which runs perpendicular to the handle, like an adze.
Although a pick axe can be used as a weapon, as Sofacoin indicates (above), it is a little unwieldy for use in close combat. Not that I've ever tried deliberately to hurt someone with a pick. Now the handle on its own would be a different matter, but the pick itself is a bit heavy for battle. Or maybe I'm not strong enough to wield it. Whatever.
Rather than causing mayhem on the battlefield, I use mine in the garden for either removing rootballs or for digging narrow trenches or breaking up old brickwork or paving.
A note on using a pick to break up a concrete slab.
A concrete slab is best broken up using the pick and a heavy sledgehammer. The technique is to first use the pick to get underneath the slab and lift it. Once it is raised slightly, put a chock or wedge underneath the raised slab and then start using the hammer. The concrete will break easily.
If you just hit the slab with a hammer, it won't break (except with a great deal of very hard work), as the soil underneath will support the concrete. By raising the slab slightly, the hammer can apply a load which puts the lower part of the concrete in tension. Concrete is strong stuff, at least in compression. It is fairly weak in tension. So when you hit the supported slab, you are applying compression to the top of the slab. Once it is raised off the supporting base, the slab bends when hit and this puts the bottom part in tension, which then cracks, and the crack then propagates through the whole thickness of the slab.
This, incidentally, is why reinforced and pre-stressed concrete is so strong. Reinforcement bars are positioned to take the tension load, while pre-stressing applies a heavy compressive load throughout the concrete. A bending load on top that would normally apply a tension force merely reduces the compressive force somewhat, leaving the load in compression.
rootbeer277 notes that when using it to dig rocks, if you hit a pipe or other hollow object, the noise will be a resonant echo. Hitting rocks and other solid objects will result in a more or less silent strike. So the advice is, first to check for pipes, cables and so on in any area you are using a pick. And second, if you get a hollow sound, then stop and use a more sensitive instrument to find the source of the echo.
Swung properly, a pickaxe will smash through almost anything found in the garden. That includes roots and concrete, but it also includes flesh and bones. Steel-toed workboots are helpful, but more sensible is careful use of the pick. Never, ever swing the pick in a way that it is likely to hit your own body. Swing the pick so that it makes contact with the ground or the plant at the end of the swing. However, stand with legs apart so that if the swing goes wrong for some reason, then the business end will pass safely between your spread legs. Second, if you are tired or fatigued, then stop using the pick.
Also, note that the two ends of the pick have different designs. One is a point, designed for deep penetration, or intensive breaking operations. The other is a blade designed to cut through things and give leverage. Use the appropriate end for the task in hand.
Protecting your back
Using a pick incorrectly can put a large strain on your back. I know this from personal experience. If you are used to swinging a heavy axe, or a large hammer then your lower back muscles might be strong enough to cope with a pick, but a pick is a heavy instrument and many people won't have developed the right muscles to swing it repeatedly.
However, a pick also gives rise to risk of strain as it is used like a lever. An ordinary axe or hammer can be swung and the impact marks the end of its action. However, a pick is designed to be used as a lever. Swing it, bury it in the soil, or under a concrete slab and you then have the opportunity to use the handle to lever up the concrete, or to break up the soil. It is this action which runs the greatest risk of straining a back muscle.
Follow the advice, keep a straight back. Use your thighs and arms to apply pressure; keep your back straight, both when swinging it and when using it as a lever.