A service is a job or work done for someone else. All the service trades form a service economy.
Good examples are:
The old split between product and service is now often a service–product continuum.[1] Many products are being transformed into services.[2] An example is IBM, which made computers, now is mainly a consultancy for businesses which use computers. That has been so at least for the past 40 years.[3]
The person or company which gives the service will get something in return for the service, obviously. Who gives the service usually gets money in return. Who gives the service may get goods in return. Who gives the service can get another service in return. This is a type of trade.