Civilians are encountered on Terror Missions (X-COM: Enemy Unknown) and during alien surface attacks (X-COM: Terror from the Deep). As mentioned in your briefing, "You must attempt to save the lives of any civilians in the area..." Civilians seem to lack any form of AI, which makes this more difficult. Instead of running away from their alien attackers, they will instead walk around as if on a Sunday stroll, sometimes even directly into the crossfire.
Aliens will shoot at civilians and soldiers with equal ferocity. Alien attacks on civilians will give clues as to the alien's location. Civilians attacked by Chryssalids will be turned into Zombies, which can result in a "Chryssalid apocalypse" scenario!
All civilians encountered during a mission will be of the same type (though this can be changed in OpenXcom). You will not be able to find men and women together in the same locale. All civilians are unarmed and have the same stats. We don't know where they parked their cars or why only the members of the opposite sex sought fit to flee by themselves.
Tips on Dealing with Civilians
If you don't care about score, then let the aliens have their way with civilians. While the aliens shoot at civilians, they aren't shooting at you. But that can mean losing up to 480 points.
If you do care about score, or if there are Chryssalids around, then your best bet is to stun any civilian you encounter. This takes them out of the picture as a glitch puts them under alien control. However, at this point they are no longer worth points to either side in the conflict. The same happens when using mind control on civilians. See why civilians go rogue for more information.
When X-COM arrives at a terror site, there may be as many as 16 Civilians still alive. However, there are occasions where the aliens finished off all the Civilians before you got there (and are likely engaging in wanton property damage before leaving.) On average, expect between 8-16 Civilians still alive when you arrive. In Ship Rescue Missions, there will be 0-16 Civilians on each stage (on deck in the ship's hold).
Keep in mind that X-COM is only penalized for civilian deaths that occur while X-COM is at the site. Furthermore, should X-COM fail or abort a ship rescue mission during the first stage, X-COM will not be penalized for the civilians below decks (the game will not generate them until you reach the second stage, and therefore they will not count as killed).
Civilians have a neutral allegiance. However, if Mind Controlled, they will turn to the Alien side on the next turn. They even count as an Alien kill on your end-of-mission score count, however, you gain 0 pts.
Action Score ------ ----- Each Civilian saved +30 Each Civilian killed by Aliens -30 Each Civilian killed by X-COM -50
TUs 35 Health 30 Stamina 65 Reactions 30 Strength 20 Bravery 80 Firing Acc. 30 Throwing Acc. 50 Melee Acc. 50 Psi Skill 0 Psi Strength 5 Energy Rchg. 20 Armor 0