Point buy

Point buy system
The point buy system is one method of determining ability scores in the popular role-playing game Dungeons & Dragons, versus using a predetermined array of scores or rolling with dice. Using the point buy system lets the player removes the element of randomness present in rolling ability scores and provides more flexibility than the predetermined ability scores.

In the point buy system, an ability score is a number between nine and eighteen with a certain point cost affixed to it. A player has a certain number of points to spend on their ability scores; a campaign will have a higher point buy (more points available to spend on ability scores) the more powerful the characters are intended to be. (Characters are usually more powerful for a more difficult game.)  Typical points allowed (from Dungeon Master's Guide: Low-powered campaign, 15 Challenging, 22 Tougher campaign, 28 High-powered campaign, 32 (most common)

Standard point buy: Ability score point costs
9 - 1 point 10 - 2 points 11 - 3 points 12 - 4 points 13 - 5 points 14 - 6 points 15 - 8 points 16 - 10 points 17 - 13 points 18 - 16 points

The point buy system, as well as other methods of determining ability scores, is discussed on page 169 of the Dungeon Master's Guide.