Strength is a character's physical power. The d20 SRD defines Strength by "Strength measures your character’s muscle and physical power."
Alternate names: Physique, Muscle (Toon).
Human Gender Differences in Strength in the Real World
Men and woman, on average, possess slightly different characteristics in how their muscles develop. However, men have significantly greater muscle mass, especially in the upper body. In most RPGs, the differences are too trivial to define categorically. However, in some RPGs the distinction might be useful to make if the game aims for great realism and the game system is able to make very specific distinctions. Defining gender differences raises the issue of which gender to use as the baseline.
In D&D Third Edition terms, gender differences probably account for one point or less of Strength (usually less than +1 point of modifier). However, men might realistically enjoy a two or three point advantage for the purposes of encumbrance, grappling, and throwing objects.
Because of the necessarily abstract nature of attributes, "realism" can be a difficult test to establish.
What Strength Really Measures in an RPG
In D&D, Strength affects a character's performance in melee combat, thrown weapon damage, lifting and encumbrance, and skills such as Climb and Jump. Because it controls so many athletic activities, Strength might be thought of as encompassing power, "twitch muscle," muscle memory, and agility. Lifting and encumbrance is probably one of its least important functions, rarely referenced in game, so Strength can be considered as only secondarily brute lifting power. Relatively Large or Small creatures have much more significant differences than do characters with different Strength scores.