The primary difference is the precious metal content. 9ct gold contains 37.5% pure gold. 18ct gold contains 75% pure gold.
The remainder is an alloy of various metals, which may include copper, zinc, silver, palladium or platinum, depending on whether it’s a yellow, rose or white gold piece.
An 18ct gold piece will generally be more expensive than an equivalent 9ct gold piece, as it contains twice as much pure gold.