Dungeons and Dragons uses a simple currency system. Currency comes in the form of metal coins. The four most common are copper pieces, silver pieces, gold pieces, and platinum pieces. Other denominations exist, but they’re used less commonly.
Gold pieces (gp) are the currency which adventurers will use most commonly. A gold piece can buy a spear, 50 ft. of rope, 20 arrows, two days of trail rations, or a goat. A skilled artisan can earn roughly a gold piece for a days work.
Gold pieces are the standard measure for wealth and for the cost of items, and items available for sale are listed in rulebooks with their cost listed in gold pieces unless the item costs less than a gold piece. NPCs you encounter do most of their business in smaller denominations because most day-to-day goods like food and clothes cost less than a gold piece, but merchants typically discuss trades and large exchanges in gold pieces.
One gold piece is worth 10 silver pieces (sp). Silver pieces are the most common denomination used for day-to-day trade between commoners. A single silver piece buys the work of an unskilled laborer for a day, a flask of lamp oil, a club, a stay at a cheap inn, or other similarly commonplace things.
One silver piece is worth 10 copper pieces, which are the smallest denomination of currency. They are commonly traded among unskilled laborers, beggars, and other poor people. A single copper can buy a candle, a torch, or a piece of chalk. Adventurers trade in copper pieces only rarely, typically using them to buy cheap items like torches which they often need in large quantities.
Two other denominations of coin also exist, but are used only rarely. Platinum pieces (pp) are worth 10 gold pieces. They’re too valuable to be used by common folk, but they’re great for transporting large quantities of money, and for adventurers who commonly carry large amounts of coin in their travels, they are a useful asset.
The fifth and least common form of coin are electrum pieces (ep), worth 5 silver pieces each. Electrum pieces are frustrating to use due to their unusual value. In fact, they’re so frustrating that Strahd, the evil vampire lord of Ravenloft, forces his subjects to use electrum pieces instead of gold specifically to make their lives harder.
50 coins of any denomination weigh roughly one pound, and coins are roughly the size of a real-world U.S. quarter. People who are good at math and know how much metal weighs will notice immediately that this doesn’t make real-world sense, so if this bothers you, just imagine that the coins are alloys of some sort like real-world coins are.
If you have trouble remembering the denominations, it’s easy to compare them to real-world U.S. currency. A gold piece is a dollar, a silver piece is a dime, and a copper piece is a penny. Trading is done in gold pieces in the same way that real-world trading is done in dollars. Even though larger denominations exist, you don’t discuss prices in those denominations. You don’t buy a shirt or something for “one ten dollars” or “one platinum piece”, you buy it for 10 dollars or 10 gold, and you can pay with a ten dollar bill if you happen to have one.
Other forms of currency also exist. At extremely large quantities of wealth, characters might trade in bars of gold or platinum, or in bank documents or other less-tangible forms of money. These are uncommon day-to-day, but these forms of wealth do exist in some settings.
Some settings also include region-specific currencies. These currencies might be made from unusual materials and come in odd shapes, but they typically have and equivalent value in more common coinage.