Before stating with oracle we should know the basic technical terms.
The Oracle System ID (SID) is used to uniquely identify a particular database on a system.
SID = DataBaseName + Instance Name.
Nothing its just a of a database which is the collection of data files, Redo Log files, Control file and etc. In RAC a database can be shared to multiple instanaces.
Instanace is a collection of all the backgroud process of oracle. While startup the oracle should be start the instance first, so it includes Memory process(SGA and PGA), SMON,PMON, DBwR, etc. Once the instance is started then it'll start the database.
Usually the same as your company domain (somecompany.com).
GDN = Database name + database domain (somedb.somecompany.com).
A database service name is a service created in listener to capture the application connections. Whenever we configure listener, the service name will be given where it listens/captures the connections from client and connects to DB using service and OS/User/Password authentication
A file that has to be configured on oracle server to resolve incoming connection requests to the listener on Oracle Sever.
Afile that has to be configured on the client computer to tell the client computer the address where the listener is located.
The ORACLE_UNQNAME is an operating system environment variable that holds the database’s unique name value.
Set of environment variables for linux system.
a directory where the Oracle software is installed;
ORACLE_BASE is an environment variable used as base directory for an OFA installation.