The main ingredient in a typical pesto sauce is fresh basil. Pesto, originating from Italy, is traditionally made by grinding basil leaves with garlic, pine nuts, Parmesan cheese, and olive oil to create a vibrant green sauce. Fresh basil provides a unique aromatic flavor, complemented by the nuttiness of pine nuts and the richness of cheese and olive oil. While traditional pesto uses these classic ingredients, variations can include substitutes like walnuts or different herbs, and prepared pestas are popular for their convenience and versatility in pasta dishes, sandwiches, and salads.