Minecraft wiki entry for Ladder, associated text: If the player pushes against a wall, the player moves upward at about 5/2 meters per second instead of horizontally. (Thus inside a 1×1 ladder shaft, the player can use any movement key to move upward.)
If a player occupies the 1×1 ladder shaft, the player can ascend the ladder by holding jump instead of using a horizontal movement key.
If a player moving perpendicular to the ladder block enters the ladder block, the player begins to move upward, but then falls off the ladder shortly afterward if the player continues to move horizontally.
The player's maximum downward speed is reduced to a "descending ladder" speed.
The player's horizontal movement friction is the same as on ice. (That is, moving around in the tiny space the player is in while still hanging on the ladder at the same level.)