If you perform duties that help someone else, you're providing a service. If you make dinner for your neighbor's children while she's in the hospital, you're doing a good service.
Service has many uses, all having to do with serving someone, or doing something for them. In a restaurant the waiter provides service, and if you do something nice for someone, you've done him a good service. If you're in someone's service that means you work for her. Service can also mean a verb meaning "to fix," like when the maintenance whiz services the photocopier, or the mechanic services your car. In church, a ceremony is also called a service.