Let me help you to understand the exact difference between them.
It is a service in OSB which is exposed to source system or calling applications or services, which means, this is the service provided to the source system.
It is a service in OSB which is used to connect to target system, which means it’s used to fetch the data from JMS, DB or any other third party system etc. and provide the data to the proxy service.
Let’s me help you to understand the difference with the help of diagram