The average protein in our body is around 300 amino acids long. All proteins are built of 20 different amino acids. If you were to build each possible 300 amino acid protein you would need more atoms than exist in the universe. Furthermore, even if you were to do so, only one in a billion of these proteins would actually fold into a stable 3d-conformation. And still, all 80.000 different proteins in our body do fold perfectly stable.
A protein domain is a sequence of amino acids that is part of a protein folds independently of the remaining protein to a stable structure. You can think of it as a 'building block' for a protein. We, humans, have inherited 93% of all our protein building blocks from non-vertebrates (e.g. worms, starfish, crabs) with only 7% being specific to vertebrates. However, we have twice as many unique protein domain combinations that these non-vertebrates which in part explains our progressed complexity.
Crazy biology facts