A chromatid is one of two identical halves of a replicated chromosome. During cell division, the chromosomes first replicate so that each daughter cell receives a complete set of chromosomes. Following DNA replication, the chromosome consists of two identical structures called sister chromatids, which are joined at the centromere.
So what's a chromatid? Well, during DNA division, when a cell divides, it needs to take its DNA and duplicate it and then transfer half of it to one cell and half to the other cell. Well, DNA's arranged in chromosomes, as you know, so what happens is, as a chromosome replicates, or makes a copy of itself, it's arranged as two chromosomes next to each other, called chromatids. Then during mitosis, when the DNA is transferred to the two daughter cells, one of each of those chromatids is transferred to each of the two cells. So a chromatid is one copy of a chromosome after DNA replication.William Pavan, Ph.D.