When characters are traveling throughout the wasteland, they can randomly encounter Critters or other players traveling nearby. When you find an encounter, you might be forced into it, hence the expression "forced encounter". Other times you will get a prompt giving you the option to enter it or skip it. That option is more likely with higher Outdoorsman skill.
Finding an encounter will stop your movement on the map and flash a red lightning bolt for a few seconds, giving you a moment to prepare for probable combat. When the encounter starts, your character's placement relevant to others there is based on a Perception check. So the lower your PE, the closer you'll likely be to critters when the encounter begins.
When a forced encounter starts, your text box will list what/who else is there. If you find other players already there, their names show in red. If the encounter runs in Turn-Based mode, your character's Sequence will determine how soon he gets his turn.
It is possible to be placed in a random encounter in the middle of a mine field or on top of bear traps. It is possible to die quickly in random encounters and have no chance to even react before certain NPCs kill you with explosives. (If your encounter is starting at the same time as a server lag, this can also end in disaster). Be alert when traveling in the wasteland! If your best option is to flee, you usually have a moment to make that decision and start running for the exit grid before combat begins.
A "vs enc" is a versus encounter, which means it has more than one NPC group and they fight each other. These are popular to hunt when leveling as most of the combat risks are directed between opposing NPC groups, and thus not entirely on you. You can sometimes just stand and watch safely from a short distance as two NPC groups fight. Wait for their fight to end and wait for the winners to loot the remains, then see if they missed anything.