DNA ladder is a solution of DNA molecules of varying lengths. 

DNA ladders are used in agarose or acrylamide gel electrophoresis as a reference to estimate the size of unknown DNA molecules that are separated based on their mobility in an electrical field through the gel. 

DNA ladders are created by restriction enzyme digestion of DNA fragments with known lengths from natural sources. They are also known as molecular-weight size markers because they help differentiate DNA fragments based on their molecular weight, which then distinguishes them in terms of size.