Ladders are currently only available in Infdev and /game/.
A ladder block is created on a workbench using 7 stick blocks, arranged in an H pattern.
Climbing Ladders
Ladders are used by placing a ladder block on a vertical piece of wall, and moving into any wall against that ladder block. The player then automatically rises, as long as there is a ladder and they keep walking into a wall.
To climb vertically, a ladder block is only needed every other space. However, this looks rather ugly, so if you like aesthetics don't use this bug/exploit.
It is possible to see through the side of the wall by going into third person mode with the Secret Friday Update 6. See through bug example: [[1]]