A dock is a structure that's made for bringing boats or ships in to the shore and loading or unloading them of goods or passengers. The small dock behind your lake house might be mainly used for launching your canoe.
Smaller docks are wooden planks that jut into the water — boats can pull up and be tied or anchored there. Larger docks in harbors might have gates enclosing areas of water where ships can be repaired or cargo unloaded. When boats dock, they pull up to a dock or a pier. You can also use the verb form of dock to mean "cut off," as when a farmer docks an animal's tail or a boss docks a worker's wages.