The Nice Classification divides trademarks into two main categories: goods (Classes 1-34) and services (Classes 35-45). The distinction matters for how your trademark is protected and enforced.
Protect physical products and manufactured items. Examples include chemicals (Class 1), pharmaceuticals (Class 5), software (Class 9), clothing (Class 25), food products (Classes 29-34).
Protect intangible services. Examples include advertising (Class 35), financial services (Class 36), construction (Class 37), telecommunications (Class 38), education (Class 41), technology services (Class 42).
Companies that both manufacture products AND provide services should file in multiple classes. For example, a coffee chain needs both Class 30 (coffee/tea) for products and Class 43 (catering services) for its cafes.