Pickups refer to any objects which are collected by simply walking into them.
Pickups in GTA games range from short to long term beneficial objects such as Weapons, Health, Armor, Clothing and Police Bribes, to collectibles required for 100% such as Hidden Packages and Oysters.
A Power-Up specifically refers to a pickup that provides a temporary extra ability or effect to the player. An example of this is the Adrenaline pickups in Grand Theft Auto III and Grand Theft Auto: Vice City.
Pickups disappear once the player has collected it by walking into it, unless the player fails to collect the item. The one exception to this is the dollar-sign pickup allowing the player to collect money earned on Assets.
When an armed person is killed by the player, he leaves behind the weapon as pickup for the player, which, excluding GTA IV and GTA V, floats in mid-air. Money can be collected from NPC killed by the player. The pickup can come in the symbol of "$", or stacked-up notes. Money taken from normal NPC is usually not a lot, but collecting from a planned killing in certain missions may leave a huge sum of money, which, in GTA Chinatown Wars comes in the form of a blue "$" symbol. Killing of people who recently visited an ATM in GTA IV can leave a huge sum of money in one area.
Health, armor and police bribes (3D Universe) are scattered throughout the cities in the form of a green cross or red cross, blue or golden armor symbol or a star respectively. They usually respawn at the same location after being collected.
In certain missions of a game, the mission may require you to pickup a certain item which has its own pickup symbol not found in normal circumstances. Such include wallets or documents, depending on the mission.