Every cell in your body has a sex chromosome that determines whether you are male or female. People who have the xx chromosome combination develop female characterisitics, while people who have the xy chromosome combination develop male characteristics. The sex chromosome determines your physical sexuality along with all the typical characteristics that are associated with that sex, such as more muscle mass for men, and breast development for women.
Sex cells are another matter. These are not somatic cells, they are specialized cells created solely for the purpose of reproduction. They are produced by a process of cell division called meiosis, which produces sex cells with a haploid number of chromosomes. This is fancy talk for saying the chromosomes number are split in half. The full count of chromosomes for humans is 46, so the half count would be 23. It is important each one of these sex cells, sperm for men, ova for females, have exactly half the number of chromosomes. When an ova, an egg, is fertilized by a sperm, you get 23 + 23 chromosomes, which would equal 46 chromosomes and the start of a brand new person!
The difference between sex cells in humans would be whether an X or a Y chromosome was present. Sex cells are also known as gametes. Besides the fact that meiosis, a reduction division produces haploid sex cells in the testes or ovaries, only one sex chromosome is present in the gamete. Through fertilization, the fertilized egg or zygote, will have two sex chromosomes once again. Males are heterozygous for the sex chromosomes. That is because they have an X and a Y chromosome. Females are homozygous because they have two X chromosomes. Therefore, when making gametes, males can either make a sperm that has 22 autosomes and a Y chromosomes, or they can make a sperm that has 22 autosomes and an X chromosome. Females will always produce eggs with 22 autosomes and an X chromosome. Therefore, to answer your questions, sex cells or gametes differ by which sex chromosome they contain, X or Y.