Answer :

Chromosome:
1. Chromosomes are single physical structure and hereditary traits, which are transmitted through them.
2. The structure of chromosome is best studied at the metaphase, which possesses two chromatids held by a single centromere.
Chromatid:
1. Each chromosome consists of two symmetrical strands called chromatids, formed during S-phase.
2. Each chromatid is half of the metaphase chromosome. Anaphase chromosomes have only one chromatid, due to the splitting of centromere, which becomes the chromosome.

shanju
Chromosome = one length of DNA which contains genes. It would look like this I 

Chromosome pair = Within normal cells we have two copies of every chromosome, one from mum and one from dad. The genes on the pair (homologous pair) are the same but can be in different forms e.g one can have the allele for blue eyes and the other for brown. They would look like this I I 

Chromatid = Just before cells divide, the DNA replicates itself. The duplicate chromatid is still stuck to the original by the centromere. Each chromosome would look like this X which conists of the original chromosome (now called a chromatid) and the copy chromatid. When chromatids separate and are on their own, they are called a chromosome again