Difference between Global DB name and SID

Per documentation:


The SID is a unique identifier that you can use to refer to an instance of 
the database that is available on your system. Use the SID field to specify a 
unique identifer for the default instance that will correspond to the starter 
database -- "oracle" is a common choice.


A database global name is the complete name of the database, including the 
database's encompassing network domain. The database's global database name 
has a format of db_name.domain_name. The db_name portion of the global name 
must be 8 characters or less and consist of alphanumeric characters, and it can 
contain special characters _, #, and $. The domain_name portion of a global 
database name must be 128 characters or less. (ie. oracle.animatedlearning.
com -- default SID is oracle & domain is animatedlearning.com).