Since the game hasn't been released yet, all we know about weapon upgrades come from the beta. For everything we know and can speculate about, [url=http://destiny.wikia.com/wiki/Destiny_Wiki]look at the Wikia page[/url]. From what we saw in the beta, the weapons themselves are fully made, but each one has a series of upgradable features that change how a weapon can function. This includes changing the sights, lessening recoil, adding more rounds to a magazine, and so on. Some of the legendary or higher level weapons will have better and more interesting perks. For example, one that I saw, which was a PlayStation exclusive, had one round in each magazine that does a considerable amount of damage in comparison to the rest. It's hard to say a lot about all of the weapon upgrades without diving headlong into speculation, but it suffices to say that you level up your weapon by using it and how you upgrade it (as in which upgrades do you use) are up to you. You pay a small amount to unlock earned upgrades (as in you earn it, it becomes available, and you pay to make it usable, but it's always about 100 glimmer from what I remember).