Primary metabolic pathways are present in all cells, and constitutively active. This allows the production of primary metabolites that are important for the cell’s survival and form the starting material for secondary metabolic pathways. Primary metabolites are products from the main metabolic pathways, and are integral to the organisms survival (including nucleotides, sugars, amino acids, …)
Secondary metabolic pathways are typically inducible, and have specialised functions. Different pathways can be present in different cells - this can be controlled through cell differentiation. Secondary metabolites tend to be more unusual products, including terpenes, antibiotics, and pigments.