From the Rulecard:

* D-O-O-M Lanes: Advance bonus multiplier.

* H-I-T Targets: Light all to power up Jet Bumpers and activate Award at the lock. Powered bumpers give massive scores.

* Item Mode: When the lanes are blinking, hit to get items. Open Massacre, activate Armor Kickbacks or get unlimited ammo.

* Frag Lane: Collect Frags to start Barrels o' Fun mode. Barrel targets pop up on the table, drop them as fast as possible.

* Cyber Demons: Hit Shell Targets to load the shotgun, then hit the ramp to shoot the Cyber. Kill Cybers to light Powerups.

* Powerup Hole: Collect lit Powerups and activate Item Mode.

* Massacre: Get the ball in the Massacre room and try to get a high kill-rate.

* Lock: Light Lock on upper playfield. Lock three balls for Multiball, and shoot for the Jackpot at the Icon of Sin.

* Doom 2 Monsters: Drop all targets on upper playfield to kill the Monsters and activate Boss Modes.

For a nice, illustrated overview of the rules, look at Flyer.jpg.
For the complete rules, look at Doom2_Rulesheet.txt