hmmm....

Well, if my memory serves, the Ork Boys box doesnt come with all the options, and only comes with choppas, sluggas (the pistols), and shootas (the.. uh.. guns)

The Big Shoota is commonly modified by taking two shootas, one with the bullet chain hanging out as the big shoota stock, and then another gun barrel from another gun chopped off and added to the end (I like to put the crosshairs on the end of mine, as well), giving you a shoota thats 1.5 times as long.

And, unless you want to buy extra bits to have official weapon representations.... then you have to custom make your burnas and rokkit launchas as well.. to my knowledge. I guess you could buy metal burna boys, but... ugh! I think I'll wait the year till they release new plastic orks (hopefully) with the new codex.

But hey! the point of Orks is that they dont have manufactured armories, that stamp out identical weapons by the thousands, and the more clap-trap your weaponry looks, why! the more ORKY it looks! I have seen great Ork rokkit launchas consisting of a slingshot with a SM missile in it (sling pulled the wrong way to represent their BS).

AS for SM, theyre pretty self evident:
Heavy Bolter: the big big bolter gun with the ammo belt and backpack
Plasma Gun/Pistol/Cannon: These all have oval barrels, with vent slits around the tip, and a 'radiator' grill on the top
Flamer: the thing with the flamethrower burner tip on the end
Melta: the one with the long round barrel, thats not a plasma or a bolter
Missile launcher:.. umm.. the bazooka

 

There is also a powerful but dangerous weapon in the Ork Wargear called the Kustom Mega Blasta (a short ranged plasma cannon).

There are also weapon upgrades that can be added to the shoota, such as; shoota/rokkit-launcha combi-weapon, shoota/skorcha combi-weapon, a rapid firing upgrade, a higher strength upgrade and higher AP upgrade; all of which can be shown by building the shoota with extra/larger/longer barrels/muzzles/magazines/ammo belts.

For close combat there are "hand weapons", represented by a multitude of knifes, blades, hammers, spiked clubs, bits of pipe etc. The two main weapons used though are large axes called the "choppa" and the "Uge Choppa", some of which can have chainsaw-like edges.

Powerful Orks are sometimes equipped with large mechanical claws called powerclaws, Mekboys sometimes use cutting torches/flamethrowers called burnas, Slavers can use a grabber on a pole, while Painboys sometimes use a large poisonous syringe.

Orks can also be fitted with an insane range of bionic arms holding built in swords, drills, spikes, pistols etc.

Some orks also train "squigs" to follow them into battle. These look like big space hoppers with teeth and join in with close combat.

As Calaban said, there are no standard weapon types - each ork is armed with something built from scrap, spare parts and bits torn off dead opponents. It is this improvisation that makes modelling orks so much fun. You can desigh what your weapons will look like and how frightening they will be. For example, you could model all your big shootas as chainguns if you wanted, or as a cluster of shootas on a trolley - your imagination and your modelling skills are the only limits.

Oh, and in the spirit of fair play, it is best to have explained what every weapon is before the game starts.