Ah, as far as "noobs" go, you're asking some pretty important questions, FP.
Spies are expensive: $10,000 to recruit one, plus additional daily upkeep that varies based on the type of spy. That means they are especially dear in the early part of the game when most players run short on available cash.
Two sabotage spies in the same province equal two potential sabotage attacks, and one additional sabotage attack for each additional sabotage spy. Note that you can have different types of sabotage -- economic and military -- in the same province at the same time.
Also note that there are counter-espionage spies who hunt and kill enemy spies of all kinds.
My personal favorite, which I start to use in the middle game when I begin to have extra cash available, are intelligence spies. They capture information on units in and around the province in which they are located, and sometimes intercept communications between players.
Of course, the immediate-results espionage functions are tied to gold spending, such as instant sabotage functions, anti-morale effects, and the "reveal all armies" spy function.
Personally, I tend to avoid using sabotage spies because I find them to be expensive, only somewhat effective, and they inevitably lead to retaliation against your own industrial facilities. I do use intelligence spies, and using sabotage spies calls attention to the fact that there are spies present and increases the risk of the intelligence spies being caught. I also use counter-espionage spies as needed.
