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A protein domain is a self-contained section of a protein that is capable of independently folding into its three-dimensional structure. 

There are limited ways protein domains can fold since certain specific three-dimensional arrangements of alpha helices, beta sheets, and loops are more energetically favorable than others.  There are tens of thousands of different proteins, but there are less than 1500 known domains because similar domains appear in many proteins.

Proteins with short amino acid sequences will likely have only one domain, but the majority of proteins contain multiple domains with distinct functions.

The multiple domains of a protein work together to allow it to perform its role, and often one domain acts to regulate the function of another domain. For example, when a ligand binds to the ligand-binding domain of a receptor - this can trigger the enzymatic function of the catalytic domain to be enhanced.

The protein domains have evolved as modules that can be rearranged to create new proteins with unique functions in a process called domain shuffling. These units will combine in different orientations depending on the location of the N- and C-terminal tails of the a