The X chromosome contains a lot of genetic material and is much like a lot of our other chromosomes. It is fairly large and is essential for all humans. Females have 2 x-chromosomes but, with humans, one of these is switched off in all cells so that you don’t end up with double the amount of expression to males (as they only have 1 x chromosome).
Interestingly, this is why you get female calico cats that have patches of different colour all over their body. Different cells in their body have 1 or the other x chromosome switched on- so they get different hair colour if the x chromosomes have different versions (alleles) of the genes.
The Y chromosome, however, is present only in males and is very small. It doesn’t encode a large selection of genes- only really the ones that instruct the developing baby to become male- it is basically the instructions to make the differences between a male and a female. If something goes wrong during development, and males do not get the Y chromosome, they only have one X chromosome. These people are known as XO and develop as female as they do not have the instructions to be male.
X and Y chromosomes dictate the sex of a child. If you have two X chromosomes then you are female and if you have an X and a Y you are male. There are also some variations with more than one copy of a chromosome, for example females with 3 Xs – triple X syndrome or men with XXY or XYY.
As Loren says, the X chromosome is large and contains a large amount of data and the Y chromosome is small and only contains the data to make the child male.
Loren and Jo mentioned a key point, that the Y chromosome carries the instructions to make you male. Essentially, you can even boil that down to a single gene on the Y chromosome called SRY (which stands for “sex-determining region on the Y”). That gene switches on other genes, which eventually leads to the development of the testes.
Very, very rarely, fathers pass on their Y chromosome to their offspring and the SRY gene gets lost along the way. The baby will have and X and a Y (so it’s genome will be male) but it will develop and look like a female, a condition called Swyer syndrome.
Like Loren said with calico cats and their colouring, striping happens in human females as well. These stripes are formed when the foetus develops in the womb and is caused by different X chromosomes being turned on in different cells. Although you can’t see them, you can test to see when of the 2 X chromosomes are turned on and that’s how scientist have worked out that human females have ‘stripes’ too.