Indirect:
Indirect takes the text and tries to convert it to a valid address
eg: =INDIRECT("Sheet2"&"!"&"G6")
It can be used where ever an address is required,
like =Sum(Indirect("Sheet2"&"!"&"G6:G10"))
Address:
Converts supplied Column and Row numbers to a valid address
=ADDRESS(10,5) will return $E$10
=ADDRESS(10,5,,,"Sheet2") will return Sheet2!$E$10