As my fellow gamers have mentioned, a survivor is a lady in a blue suit with a blue cross above her head. She appears twice on each level. Whenever you hear her scream for help, locate her, touch her and fight off zombies until she is rescued by a chopper (that should take about 10 secs).
A few tips:
1. You'll have to solo it (or make a pact with your squad mates that they do not attempt rescuing) so that each survivor will be credited to you. You won't make it with the four initial lives, but don't worry, as:
2. You are allowed to lose all your lives and then continue, and the survivor count will not reset. So you'll probably have to do quite a bit of dying for this achievement.
3. But please do not quit the game entirely. If you quit out and then continue, you'll have to do it all over again. So, save all the 25 in one sitting.
4. Use your Teddy Bear to lure zombies away from the survivor. This will make rescuing much easier, and as soon as the survivor is picked up by a chopper, they drop down a new Teddy Bear for you (and you are allowed to carry only one at a time).
Hope that helps.