Shooting enemies will reveal power-ups and continually shooting them will either change their type or reveal a special weapon.
Different weapons are more effective than others when fighting the end bosses.
You have the ability to hold and select between five different weapons at a time. Each weapon can be powered-up a couple of times by collecting a similar option. When you lose a life, you don't lose all of your weapons, just the one you are using when hit. Don't set yourself up for a great ending when you complete the game. There really isn't one.