CRITICAL BUG: This is nothing but a crash...suppose when you are trying to play a song from media player of MIDI format,or when a incoming call comes to your mobile when u r testing a particular feature in your device it may crash.They r many EXAMPLES like this.
BLOCK: Unable to boot your device or unable to go back to previous screen or unable to go to ur main menu when you are testing particular feature.unable to invoke anything untill and unless you reboot your device
Major: Unable to funtion or misfunctionality of a particular feature 
Minor: Usually GUI bugs comes under minor bugs. These bugs wont AFFECT functionality. Example overlapping of letters in MESSAGE COMPOSE screen.

CRITICAL BUG: This is nothing but a crash...suppose when you are trying to play a song from media player of MIDI format,or when a incoming call comes to your mobile when u r testing a particular feature in your device it may crash.They r many examples like this.
BLOCK: Unable to boot your device or unable to go back to previous screen or unable to go to ur main menu when you are testing particular feature.unable to invoke anything untill and unless you reboot your device
Major: Unable to funtion or misfunctionality of a particular feature 
Minor: Usually GUI bugs comes under minor bugs. These bugs wont affect functionality. Example overlapping of letters in message compose screen.