There are two types of silver, unbound and bound. The first type that you are introduced to in the game is bound.
You get bound silver from quests, level up packages, and events. It can only be used to buy things from NPCs and can't be used to buy from other players in the market. It is used to buy things like potions and pet food.
Unbound silver is the primary resource in the game. It is used to buy things from other players in the market and can also be used to buy things from NPCs. You can get it from all dungeons that are level 45+.