Short answers, pulled from the story.
A protein is a large biomolecule made of one or more long chains of amino acid residues bonded together by peptide bonds. Proteins perform a vast array of functions in organisms, including catalysing metabolic reactions, replicating DNA, responding to stimuli, providing structure, and transporting molecules.
The term protein was proposed in 1838 by the Swedish chemist Jöns Jacob Berzelius, an associate of Gerardus Johannes Mulder. It derives from the Greek word proteios, meaning primary, in the lead, or standing in front.
A protein contains at least one long polypeptide chain and usually a stable 3D structure, while a peptide is a short amino acid oligomer often lacking a stable shape. The boundary between the two is not well defined and usually lies near 20 to 30 residues.
Proteins have primary structure, the amino acid sequence; secondary structure, local repeating shapes such as the alpha-helix and beta-sheet held by hydrogen bonds; tertiary structure, the overall shape of one molecule; and quaternary structure, several chains acting as one complex. A fifth level, quinary structure, describes the surface signatures that organise the crowded cell interior.
Insulin was the first protein to have its amino acid chain sequenced, done by Frederick Sanger in 1949. His work proved that proteins are linear polymers of amino acids, and he won the Nobel Prize for it in 1958.
The largest known proteins are the titins, a component of the muscle sarcomere. Titin has a molecular mass of almost 3,000 and a total length of almost 27,000 amino acids.
The most abundant protein in nature is thought to be RuBisCO, an enzyme that catalyses the incorporation of carbon dioxide into organic matter during photosynthesis. Plants can consist of as much as 1% by weight of this enzyme.