Sex chromosome

Sex chromosomes are chromosomes which determine the sex of individual organisms. In men, for example, there are 23 pairs of chromosomes, and one of these pairs are sex chromosomes. Females have two X chromosomes, males have one X and one Y. An egg always carries a single X, while sperms carry either an X or a Y. That is how sex is determined in humans.
Not all organisms have their sex determined by chromosomes. Those that do use the sex chromosome system have variations in how it happens. The account above is generally true for mammals.