A Service is a representation of the underlying live service you are mocking in a Mock Service. Services can contain all Transactions uploaded for that live service, while the Mock Service might only require a subset of those Transactions needed for the current round of testing.
As BlazeMeter evolves, Services will increase in importance and usage. For now, consider them as a conceptual entity meant to help you organize Transactions by their place in your application architecture.
For more information about Services, Mock Services, and other key concepts, see Introduction to Mock Services.