When a single strand of DNA duplicates itself, it forms the shape of an X. It's almost as if one side of the X is a "mirror image" of the other side of the X. These two identical strands of DNA join together at the centromere. A duplicated strand of DNA is referred to as the chromosome. However, a chromosome can also be a linear shape, as a result of the DNA NOT duplicating itself. When looking at a chromosome in the shape of an X, ONE side of the chromosome is called a chromatid. Two sister chromatids come together to make a chromosome.
A chromosome is the thread - like strand that is housed in the nucleus of a cell and carries genetic information. It is composed partially of DNA (deoxyribose nucleic acid). This DNA looks like a double helix. It is made of a sugar-phosphate backbone (sides of the ladder) and nucleotide base pairs as its rungs (adenine and thymine, or cytosine and guanine). The DNA within a chromosome is tightly wound around histone proteins. Each species has a unique number of chromosomes. For example, humans have 46 chromosomes.
A chromatid, on the other hand, is a replicated copy of a chromosome. This occurs at the beginning of cell division. Thus, a chromosome is made of two sister chromatids. The two copies are joined in the center by what is called a centromere.