Allies, of course, are those people who have either equal skill, complementary skill, or are just plain the sort of people who you want in your corner when the situation starts to get sticky. They can be partners, backup, or even semi-neutrals off to the side offering the choice bit of advice (perhaps while drinking those drinks with the fruit on sticks). But they're trained professionals themselves, generally, and know how things work. While they may not be depended upon to be involved, whatever level of involvement they do have will be dependable.
Clients are those you're helping out, usually the people caught in the sticky situation. They can be of any skill level (from master+ to nonexistant), but they are the one who you are Assigned to help. A Trainee-Client (a Junior) tends to start out as a Civilian, but will with all good luck proceeds to become an Ally.
Civilians are those who are either not part of a situation, or must be shielded from a situation, but are not actively involved in it. They largely do not tend to have high skill levels, because the community of high-skill individuals is relatively small, and if someone of high skill comes close enough to the situation to be classified, they will tend to fall into one of the other categories.
Liabilities are those who tend to either actively or passively make a situation far more complex than it need be, though usually not maliciously. They could be the cause of a situation, or they could be only a complication, but whatever it is that they are, they tend to be right in the way doing the wrong things at the wrong times. Some Liabilities become your Clients, especially when end-user education to make sure that this person is not a Liability again becomes necessary. If they don't turn into yours, they should probably be someone's Client, just to keep loose cannons under control.
Adversaries are not only in the way and making the situation worse than it ought to be, they're often the cause of a situation, and they're doing it on purpose and it's generally malicious. Adversaries generally probably ought to be someone's Client, just as Liabilities should be, even if they never become yours. Adversaries do not, as a rule, intersect with Allies.