Chemistry is all about electrons. Electrons used to go by old-fashioned names like corpuscles and were once even thought to move like fluids in an ether. So chemistry is really all about atoms and molecular bodies exchanging precious electrons.
Most garden variety chemistry is unimolecular or bimolecular. Unimolecular chemistry is dissociative, which on the one hand sounds asocial but on the other hand really just means to come undone. Bimolecular chemistry is more about two things coming together. Molecular threesomes do occur but they are rare (as one can imagine) and are technically called termolecular reactions. Concerted orgies of four or more molecular bodies are mostly just pure fantasy, unless there is some sort of prior bondage involved as sometimes occurs amongst biomolecules. We'll save that for another day and focus on just the basics.
When two atoms or molecular bodies get together to react, there's always a donor and an acceptor (of the precious electrons). Now besides the obvious connotations of donor and acceptor, it also turns out that the electron donor and acceptor parts of molecules have certain shapes. You may have also heard tell about HOMO's and LUMO's but that's getting a bit too technical. We'll save that for elective or advanced classes. Suffice to say that some molecules can be both donors and acceptors depending on the shapes and energy levels of the coupling partner(s).
Many of the concepts I've just introduced can be illustrated graphically. One of my personal favorites is the simple coupling of H2 with O2 to make water (or steam). But it turns out that this simple reaction is "forbidden" and so unfit for the present discussion. Meanwhile, here's a different diagram to wet your appetites: