Staircase

A staircase is represented by either a < or > character, depending on whether the stairs go up or down. A staircase will take you to another dungeon level above or below you. A few of them are special:

The level 1 staircase
This is the only staircase in NetHack which can potentially go to two different locations. If you climb it without the Amulet of Yendor, you escape the dungeon and the game ends. If you have it, you enter the Plane of Earth.

The vibrating square
The vibrating square is actually a hidden down staircase that can only be revealed by performing the invocation ritual.

Ladders
The staircases in Vlad's Tower are actually ladders. They behave for all purposes exactly like staircases. Ladders can also be found on some quest levels.