Diploid

In most species chromosomes come in pairs. These species are called diploid. The two chromosome of a pair are homologues. Homologues means that they are the same type of chromosome: they have the same genes at the same loci. Please remark that this not mean that they are identical: a gene on the one chromosme can be a different allele than the gene on the other.

So an individual from a diploid species with 3 chromosome pairs you could represent like:
A A'
B B'
C C'

So diploid means that every gene is present twice: once on each of the chromosome in the pair. There is one exception to this rule and that are the sex chromosomes. The sex chromosomes are better know as the X and Y chromosome.

In birds a male has two X chromosmes so he adheres to the rule of chromosome pairs. But the hen has one X and one Y chromosome. These X and Y are only for a small part similar (homologue). The Y chromosome is much smaller and contain almost no genes.

(Note that in humans XX is a woman and XY is a man)