Chromosomes are structures of nucleic acids and protein carrying genetic information in the form of genes.
The chromosome is composed of two sister-chromatids.
During meiosis and mitosis, the sister chromatids are separated to separate half of the geentic material and create a new cell.
The difference between sister chromatids and non-sister chromatids is that sister chromatids have the same alleles in the same place and non-sister chromatids have different alleles in the same place. Non-sister chromatids will have the same length and configuration, but those different alleles will represent different types of the same characteristic (for example: different colors of fur). But they look the same as sister chromatids.
