What are chromatids and how do they differ from chromosomes

Chromatids are only called as chromatids when they are sister chromatids, meaning that when chromosome have two double stranded DNA attached together, each of the double stranded DNA is a sister chromatid, and while the whole thing (the two sister chromatids that are attached together) is called a chromosome.

So after the DNA has been replicated, the chromosome consist of 2 sister chromatids, attached to each other at their centromere.

Overall, sister chromatid is one of the two double stranded DNA in a replicated chromosome, and chromosome is the whole "X" shape that we see. For example, in humans, after the S phase of interphase in the cell cycle, we have 46 chromosomes, with 92 chromatids, since each chromosome is an "X" shape, by having two sister chromatids.

Hope this helps!

Where are the phrases