Reactions

Rearranging, never creating.

Conservation

Atoms are rearranged, not made or destroyed, so a chemical equation must have the same count of each element on both sides. Balancing is bookkeeping for atoms.

Combustion, neutralisation, oxidation and precipitation cover most school chemistry.
A reaction that releases energy is exothermic; one that absorbs it is endothermic.

Rates

Concentration, temperature, surface area and catalysts change how fast a reaction goes. A catalyst lowers the barrier and emerges unchanged, which is what enzymes do in the body.