(3) Short arm
(4) Long arm
A chromatid is one copy of a duplicated chromosome, which generally is joined to the other copy by a centromere, for the process of nuclear division (mitosis or meiosis). They are normally identical ("homozygous") but may have slight differences in the case of mutations, in which case they are heterozygous. They are called sister chromatids so long as they are joined by the centromeres. When they separate (during anaphase of mitosis and anaphase 2 of meiosis), the strands are called daughter chromosomes (although having the same genetic mass as the individual chromatids that made up its parent, the daughter "molecules" are still referred to as chromosomes much as one child is not referred to as a single twin). Before replication, one chromosome is composed of one DNA molecule and after there are two DNA molecules. This is because DNA replication increases the amount of DNA and does not increase the number of chromosomes.
In other words, a chromatid is "one-half of two [normally] identical copies of a replicated chromosome". The two copies may have slight differences due to mutations. A chromatid is simply a copied chromosome which is paired with a (normally) identical chromosome at the centromere. It should not be confused with the ploidy of an organism, which is the number of homologous versions of a chromosome.