Both the tools are quite similar but have few differences.
Few of the differences are -
- Power Bi Server can be on-premise in companies or on cloud whereas Service is always on cloud.
- Gateway is required to connect to data sources for Service but not required for Server.
- Dashboards can be created as well as viewed in Sevice but not possible in the case of Server.
- Q&A is not available in Server
- Preview not available in the server.
There are more similar differences, find them in the Microsoft docs below