As per Oracle Glossary : SID is a unique name for an Oracle database instance. ---> To switch between Oracle databases, users must specify the desired SID <---. The SID is included in the CONNECT DATA parts of the connect descriptors in a TNSNAMES.ORA file, and in the definition of the network listener in the LISTENER.ORA file. Also known as System ID.
Oracle Service Name may be anything descriptive like "MyOracleServiceORCL". In Windows, You can your Service Name running as a service under Windows Services.
You should use SID in TNSNAMES.ORA as a better approach.

Global name is set up in your init.ora file. It can be changed later if you didn't set it up, you can search the oracle docs for that. But it's much better if you plan for it in the beginning; it's tricky to change an existing instance's global name. If you need to do this, first try it out on an old computer, then do it in your production environment.

Global name is used for remote database links. If you don't set up a global name, your database will be known as ORACLE.WORLD . This will cause problems for people that remote link their databases into yours. So when setting up an Oracle instance, you should always set your global name to include your company's domain. Example, instead of saying oracledb for your global name, use oracledb.mycompany.com .

Even if you don't allow other companies to remote link to your databases, what if you install a second Oracle server? Both servers will think they're ORACLE.WORLD. So you might want to set up your global name to be something like oracledb1.mycompany.com , or, oracle-dev.mycompany.com and oracle-prod.mycompany.com ... or accounting.mycompany.com versus datawarehouse.mycompany.com .. etc etc

-Thomas[@more@]