Protein domains are building blocks of proteins. They are reusable blocks that often represent a functional or structural unit.
The combination of domains inside a protein (domain arrangement) defines its function. Different processes can lead to rearrangements of domains and thus influence the function of a protein.
The DomainWorld program suite is a collection of programs meant to facilitate the analysis of domain arrangements and their changes during evolution.
It also contains programs to annotate protein sequences with domains but as well can handle input from standard annotation tools like PfamScan.