Difference among INSTANCE NAME , DATABASE NAME and DB_UNIQUE_NAME

INSTANCE NAME
SID and instance name are same. when we start any instance(oracle) then SGA is allocated and BG processes are started . This instance is the gateway in order to access the database which is stored in the form of datafiles on disk . We call this phenomenon with a name that is nothing but a instance name. starting instance is nothing but invoking oracle software for your own use. let me give you a example- suppose you want to watch a movie then you will invoke window media palyer first then you will access the movie file with this player.even if you don't invoke the player and directly click the movie file then also player is invoked first and then only you can watch the movie. so invoking the window media player is like invoking the instance (oracle s/w) and watching the movie is like accessing the database.The movie is stored in the form of movie file and database is stored in the form of datafiles.

DB_NAME
db_name  is the name of the database that is stored in form of OS level datafiles.  You can use the invoked instance in order to access this database . Hence SID name and database name can be different and instance name for the database can be changed  at any point of time if database shutdown is allowable. When ever you want to change the instance name of your database then export different SID and start the instance with this name and make sure that this name is included in name of your parameter file.

DB_UNIQUE_NAME
db_unique_name is just one more name(unique name) along with db_name of your database when the database participates in configuration with more than one database. This db_unique_name is used to uniquely identify the database among many databases in a network. many databases can have same db_name but can not have same db_unique_name. for example -In dataguard environment,primary and standby will have same db_name but will have different db_unique name so that each of the database can be uniquely identified while registering in RMAN as target or while adding in dataguard broker configuration.