There is a total of eight actual weapons: the Twin Shot, Back Shot, Blade, Rail Gun, Snake, Freeway, Hunter, and Thunder Sword. Shields, the Claw, and 1-ups are not actual weapons, but either cause extra destruction or prolong the time in which pain can be inflicted upon your enemies.
Twin Shot[edit]
Your basic double laser. It fires two orange streams of laser pellets and is relatively weak, but is very reliable and cannot be lost. Good for finding weak points on bosses and can be upgraded to Blade.
Back Shot[edit]
Similar to the Twin Shot, it fires a single stream of blue shots forward and backward. Also relatively weak and cannot be lost. If you do not have a Freeway, Hunter, or Rail Gun, you will often be stuck with this weapon in some situations. Can be upgraded to Rail Gun.
Blade[edit]
An upgrade to the Twin Shot, this weapon fires large orange blades forward in rapid succession. While a powerful weapon, it is not good for hitting small weak spots, as the blades will often end up detonating on the armor around the weak spot.
Rail Gun[edit]
This is an upgrade to the Back Shot, and fires two extremely powerful blue lasers straight backward from your ship. Very useful on enemies behind you. This weapon would be very good, as it fires the single most powerful shots in the game besides the Thunder Sword, but unfortunately it can only fire backward.
Snake[edit]
Fires small explosive grenades at about a 75° angle above and below you. This is a very specialized weapon and may be tossed aside by some who does not see its true potential. The small grenades are actually very powerful, and are best used on many of this game's bosses. You get no extra firepower by using the claw with this weapon, but the claw will leave shadows of itself around you, adding extra protection.
Freeway[edit]
Not very powerful, but has a wide range of effect, firing a spread of five missiles in the opposite direction you are going while simultaneously aiming your regular shots in the direction you move your ship (e.g. if you flew down and to the left, the missiles would aim to the upper-right and your shots would aim to the lower-left). Good against multiple enemies. Fires seven missiles with a claw.
Hunter[edit]
The most useful of the special weapons, it rapidly fires three streams of blue energy spheres that home in on your enemies. It is about as powerful as the Freeway in terms of damage per projectile, but the homing capabilities make it well worth the lesser damage. With a claw you can fire five streams of spheres instead of just three.
Thunder Sword[edit]
This weapon is great. It's able to kill most bosses in a few hits, and is the most powerful weapon in the game. After the Alien Battlecruiser level, if you have a claw you can shred enemies into pieces easily. Think of it as a portable giant blender. Sit around for a while and let the claw charge up (you'll see energy glowing around them) then press the fire button to let loose a sword beam accompanied by two powerful lasers firing from the claw at about 25° angles. If you're one of those impatient types, you can fire it before it is fully charged, but the beams will not be as long.
Shield[edit]
This is the absolute best item in the game. It's better than a 1-up. This handy little device will take three hits for you, basically making it worth three 1-ups. On top of that, it will let you use any special weapon you want without fear of losing it when you die. In fact, if you do it quickly enough, it will even let you pass straight through a wall for one hit off your shield.
Claw[edit]
If you have this item, two small indestructible probes will begin to circle your ship. This is the third best item in the game, allowing you to fire extra shots from any weapon but the Snake. Also, any of the small, round energy bullets (look like marbles) that enemies fire can be deflected if they hit a claw. This item allows you to use the Thunder Sword after level 5, too.
1-up[edit]
Pretty self-explanatory, these little effigies of your ship will grant you an extra life if you are skillful (or lucky) enough to nab them.