Asset is something valuable. Amusingly, the etymology for "assets" ultimately comes through Old Norman "asetz" from Latin ad satis, which translates to "to satiety". The meaning it had when it made its way into English was "enough". It was used in a legal sense: for example, if someone couldn't pay back their debt in money, but had asetz to pay it back with their property, then justice seized asetz from their property to settle the debt. So this was an adjective ("assets" meaning "enough"). The phrase was used in a rote way, so people knew the meaning of the phrase but not necessarily of each individual word. So eventually the adjective was interpreted as a plural noun, and the singular "asset" was invented to design the kind of things that were seized: valuable things. Since then, it has acquired other close meanings, such as something advantageous or useful. (Note that in modern French, "assez" still means "enough".)
Resource is something that you can use freely. The etymology also comes through Old French and ultimately from Latin. It's basically re- for again (as in many other words beginning with re-) and source which means rising up. Resource is therefore etymologically the same word as resurrection and resurgence, all these words have the same ultimate origin. So a resource is something that comes back up, like the crops growing after the winter, or the metals that you mine, or a source of water surging from the rock...
Therefore, both words are nearly synonymous especially in the context of video game stuff, but they have different meanings initially. You can consider that resources are things that you can use, while assets are things that you own. So if you rip the sprites from Doom, it might be your resource, but it's id's assets!
Volley comes from the French volée, which is past participle of voler, meaning to fly. So it's basically flight. Specifically, flight as an instance of things flying, not flight as the capacity for things to fly. Flight as in "Example Airline offers regular flights between Whateverville and Destination City." A volley designates shooting several projectiles so that they fly together. Like a formation flight. In sports, it also designate the fact of striking the ball "in flight" before it hits the ground.
Salvo comes from Italian this time. While it also designates the same thing (shooting several projectiles simultaneously) the meaning is slightly different. It's basically the same word as "salute" -- remember that Latin did not distinguish between U and V (nor between I and J). Originally, salus was Latin for "safe". (It gave us words like salubrity.) From this word meaning "safe" came the expression "salue" or "salve" which can basically be translated as "be well!" and was used as a greeting. This use as a greeting means that to salve people, or to salute them if you prefer, was to hail them. Now the military types have always loved using their weapons to hail their friends, leaders, and enemies, and everything, so as soon as the first firearms were invented they started having the idea to salute by simultaneously shooting several cannons at once. It demonstrates the army's strength and coordination -- perfect way to give homage to whoever or whatever they're hailing.
So basically, a salvo should preferably be used for a non-aggressive use of cannons; while there is no such restriction for volley.