This creates an invisible area which, when entered by the player, allows them to climb up or down.
Name (targetname) - Property used to identify entities.
The simplest way to create a ladder is to construct the visible bit first, with whatever texture you want, then copy the brush, drag it forward, and turn it into a func_ladder.
This entity can also simulate ropes (although the sound isn't quite right). If mapping for Opposing Force, consider using env_rope instead.
Ladders can also be created by changing an entity's "Contents (skin)" key value to "-16". Turning SmartEdit off and adding this key to a func_door allows to create a moving ladder (the ladder will behave strangely in it's open position, so it's best to keep it's open position somewhere the player can't reach).
When a CLIPHULL texture is used, the ladder behaves strangely and adds a speed boost when climbing.