Phytochemicals are literally defined as any chemical that occurs in plants (‘phyto’ referring to plant). But the term is generally used to refer to micronutrients contained in plants that can have a benefit to our health. Almost daily new phytochemicals are discovered or new roles for already identified phytochemicals are discovered.
As animals do, plants also contain macromolecules that include proteins that for both structural roles and as enzymes, lipids that make up cell membranes, carbohydrates that store energy and nucleic acids that serve as hereditary molecules. But plants have another large category called secondary metabolites that are not involved in these essential rolls of cell metabolism.
It is difficult to categorize these chemicals as many fall into more than one class and others don’t fit neatly into any. For this reason you will find different categories by different authors. Generally though, besides the main 4 classes of biochemicals (proteins, lipids, carbohydrates and nucleic acids), you can also find find phenols, tannins, alkaloids, glycosides, volatile oils, resins, and mucilages as important classes of phytochemicals.