Niche D20Gold is the common coin used to purchase weapons, armor, and other such equipment. In addition to gold exists platinum.
Gold (gp), is the lowest denominator of coin, weighing 1/50th of a pound. Each gold coin is worth 100 platinum coins (pp), a platinum coin also weighs 1/50th of a pound.
Other Wealth
In addition to gold and platinum, trade goods such as wheat, iron, salt, ect.. May be used for trade, though their size and weights may not be comparatively beneficial for trade to gold coins.
Gemstones can be another alternative to coins, having values usually greater than a single gold coin and ranging greatly in size and weight. Gemstones usually require some amount of appraisal to ascertain their true value, but can be used easily in trade when using more common gemstones.
Starting Wealth
Regardless of class all characters begin play with 100gp as well as a battered suit of armor and 2 battered weapons, all of which the character must be proficient in. Battered items function as a non-masterwork version of that item but may only be scrapped for 4d6 gold rather than sold at the true cost of the item.