Electron Transport Chain:A series of protein complexes that transfer electrons from electron donors to electron acceptors - like a game of "pass the parcel" where the parcel is an electron and the players are different protein complexes.
Oxidation-Reduction Reactions (Redox Reactions): These reactions involve the transfer of electrons between two species - similar to trading lunch items between two students at school.
Protein Complexes: Groups of proteins that work together for a common function - think about it as different groups of friends within the larger student body at school.