The word protein comes from the Greek word p??ta ("prota"), meaning "of primary importance."

Most lower life forms (microorganisms and plants) can synthesize all 20 standard amino acids. But humans (and animals) must obtain some of the amino acids from the diet.

When we digest protein, it is broken down in into its constituent amino acids. Some amino acids are used for protein biosynthesis (making new proteins), while others are converted to glucose through gluconeogenesis, or fed into the Krebs cycle to give us energy.

When we eat protein, it stimulates the release of glucagon--a hormone that promotes thermogenesis (fat burning) and opposes the activity of insulin.

References

Kieffer TJ, Habener JF (2000). "The glucagon-like peptides.". Endocr. Rev. 20 (6): 876–913. PMID 10605628.