There are a few categories of salvage, but the primary two are Base and Invention. Base salvage is used to create new items for your SuperGroup base like zone teleporters, and Invention salvage is used to crate new enhancements for your characters.
The drops can be magic or technology based but this is only really used to decide if you want a base item to look technological or arcane.
Different mob types drop different salvage, you can right click on any mob and select info to see a list of salvage that it drops. It will also show you if it drops salvage for any recipes in your current inventory
There are three level ranges for salvage:
Level 4 - 25
Level 20 - 39
Level 35 - 54
Yes I know they overlap each other on the levels, that is intentional.
The chances of you receiving salvage depends on the rank of mob you fight.
Minions = 8%
Lieutenants and Snipers = 10.64%
Bosses and Elite Bosses = 25%
Archvillains that drop rewards = 43.22%
Unwanted salvage can be deleted, put up for auction at a Wentworths Auction House, or sold in normal stores.