Encounters are single missions, where you can take down walkers, rescue survivors, and collect awesome rewards. On the map they have a symbol of a skull.

Sometimes you can see a survivor struggling with the walker and the symbol above the mission is a skull with a small waving person next to it, those missions are called Rescue Mission. In those missions you can rescue the survivor and take them to safety. The symbol above the mission is a skull with a small waving person next to it. Rare rescue missions feature 2 survivors and in some cases 3 survivors! Be aware that on Rescue Missions you can only receive Rescuee as reward.