what is a SID and Service name
please look into oracle's documentation at https://docs.oracle.com/cd/B19306_01/network.102/b14212/concepts.htm
In case if the above link is not accessable in future, At the time time of writing this answer, the above link will direct you to, "Database Service and Database Instance Identification" topic in Connectivity Concepts chapter of "Database Net Services Administrator's Guide". This guide is published by oracle as part of "Oracle Database Online Documentation, 10g Release 2 (10.2)"
When I have to use one or another? Why do I need two of them?
Consider below mapping in a RAC Environment,
SID SERVICE_NAME
bob1 bob
bob2 bob
bob3 bob
bob4 bob
if load balancing is configured, the listener will 'balance' the workload across all four SIDs. Even if load balancing is configured, you can connect to bob1 all the time if you want to by using the SID instead of SERVICE_NAME.
Please refer, https://community.oracle.com/thread/4049517