Concepts
Cycling concepts you should know about - do the homework and search for the details
Drivetrains
Chains: speeds, direction, quick link, half link, lubrication, chain rotation, checking wear, shortening
Bottom brackets: types (square taper, JIS vs ISO; ISIS/Octalink; Hollowtech I-II, Campagnolo UltraTorque/PowerTorque), shell width and thread types, PressFit, tools
Chainline (43.5 mm road, 50mm MTB...)
Chainrings: BCD - bolt circle diameter; direct mount, frame devices, clearance
Front derailleur: mounting, min/max teeth, top/down/both pull, trim
Rear derailleur: min/max teeth, cable pull (actuation ratio), cage length, pulleys, limit screws, Shimano clutch, manufacturer cross compatibility