Macromolecules are polymers. Polymers are large molecules of many similar "units" linked together. These individual units are called monomers.
Image credit: DOE Human Genome Program.
Form and Function
The variation in the form of macromolecules is largely responsible for molecular diversity. Much of the variation that occurs both within an organism and among organisms can ultimately be traced to differences in macromolecules. Macromolecules can vary from cell to cell in the same organism as well as from one species to the next.
Generally speaking, all macromolecules are produced from a small set of about 50 monomers. Different macromolecules vary because of the arrangement of these monomers. By varying the sequence, an incredibly large variety of macromolecules can be produced.
While polymers are responsible for the molecular "uniqueness" of an organism, the common monomers mentioned above are nearly universal.
Assembling and Disassembling Polymers
While there is variation among the types of polymers found in different organisms, the chemical mechanisms for assembling and disassembling them are largely the same across organisms. Monomers are generally linked together through a process called dehydration synthesis while polymers are disassembled through a process called hydrolysis.