It is like being a soldier who uses his leave time to do missions and build up more cash than they will likely ever see in the civilian world at the risk of becoming a man without a country. Not all mercenary ops include actual combat operations, however all include the same negative results if caught.
You almost never know who it is you are working for which is why you choose very carefully who you work with. If you are working for a government that fact will be completely hidden under the name of a seemingly insignificant corporate header. (A front company)
If your mission is specific, your equipment will be provided for. If it is an open contract type of situation such as a bounty, you're on your own and must provide your own equipment. Mercenaries do not use tents. If you can't stay in a hotel, the nearest swamp will do.
Pay: Depends on the mission or the target. I knew guys that came back with anywhere from 5 to 50k for a weeks leave time. It just depends. Are you working as a load master dropping door bundles over a secluded jungle or are you hunting individuals, or maybe disrupting communications, sabotaging piplines, etc.
4- Who? Could be anyone, a government, corporation, agency, or even a deep pocketed individual. You'd never know which is why very few who can do missions do.
5 - Yes, terrorists use mercenaries, but more often terrorists are mercenaries. Everyone has their bad seeds.
P.S. Those telling you that mercenaries work for private security companies have no clue what they are talking about. Those people are security contractors and they are now completely under the control and authority of the United States Congress. A Mercenary is not and the United States Government or the US Military will not even acknowledge your existence if caught.