A sex chromosome is a type of chromosome that participates in sex determination. Humans and most other mammals have two sex chromosomes, the X and the Y. Females have two X chromosomes in their cells, while males have both X and a Y chromosomes in their cells. Egg cells all contain an X chromosome, while sperm cells contain an X or Y chromosome. This arrangement means that it is the male that determines the sex of the offspring when fertilization occurs.
Typically in mammals, the gender of an organism is determined by the sex chromosomes. In the case of humans, this happens to be the X and the Y chromosomes. So as you may recall, if you are XX, you are female. If you are XY, you are male. And in the case of other mammals, this is often similar, although they may have different names for those chromosomes as well. What's really interesting is that you have a large discrepancy in the size of these two chromosomes, with the X chromosome being much larger than the Y chromosome. They also carry distinctly different genes. And what's also very fascinating about this is, I don't think it's really been determined exactly what it is on each of those chromosomes that make someone male or female in particular. It's kind of a combination of many of those genes. In fact, the sex chromosomes are quite interesting, not just because of what they cause or what their role is, but also because they're so different from the other chromosomes that we have in the complete human genome.