In early GTA games, money is emphasized as the key to unlocking new areas in the game, but it may also be used in various other activities. The formula was dramatically modified after Grand Theft Auto III, when money was only important for specific missions, as the completion of missions unlocked new areas instead; the former was removed entirely after Grand Theft Auto: San Andreas. Since GTA III, the primary use of money is the purchase of items and services, such as health, armor, clothing, properties (like safehouses and business) and weapons.
As the vast majority of settings in the series takes place in the United States of America, the currency unit of money is the United States dollar. The London add-on packs for GTA 1 (Grand Theft Auto: London 1969 and Grand Theft Auto: London 1961) are exceptions; as the games are set in the United Kingdom, the pound sterling substitutes the dollar symbol.
It should be noted that money throughout the GTA series comes only in stacks of paper bills as there are no single bank notes seen in the games (except when throwing $1 bills to strippers in GTA V). Cents and coins are also not in any of the games yet it is presumed they exist (You may see them referenced on billboards/signs or in pedestrian dialogue for example) but there are no items available for the player to purchase that are under $1.