DNA is not isolated inside the cells, but joined to proteins (histones) that have structural and stabiliser function.  The joint of these proteins and DNA is called chromatin. In eukaryotic cells, such as humans ones, chromatin is inside the nucleus.

During interphase (when cell is not dividing) chromatin is uncoiled. The cell has to use it continuously to can perform its vital activity. However during the cell division, chromatin is coiled to form chromosomes.

Human species is diploid. This means that cells have two sets of chromosomes. Each cell has 23 pairs of chromosomes (In total, 46). The members of each couple are called homologous chromosomes.

Each chromosome is formed by two chromatids. Each chromatid is an identical copy of the other. They held together by a point called centromere.