Spirit + Ginger Ale + Lemon Juice; served over Ice
Looking through various bartenders manuals you might find it hard to differentiate a buck from a Rickey, a Collins, or a fizz. Traditionally however there is a specific difference in that a buck should use ginger ale instead of soda water to provide its carbonation. The ginger ale is also providing the sweetening as well and thus no sugar is added. A buck then is the combination of spirit, ginger ale, and lemon juice in a tall ice filled glass.
For an example, see: