What spoil is, how to tell that it landed, why you spoil by the level of the mob and why it pays to keep dwarf girls of different levels.
The spoiler is the main supplier of all the materials and resources to the market of Aden. With a big online the manor (All about manor: sowing, harvesting and handing in crops) is ineffective, and everybody starts keeping spoilers to replenish their stocks of resources at least somehow. Of key materials I do not even speak: knocking out pieces for crafting by drop is the lot of losers.
What is spoil anyway? 
Spoil is a special skill with which you can get additional items from mobs, strictly defined for each mob. So we put this skill on the quick bar and use it on every mob we beat.
We follow whether the skill landed by the lines in the system chat: in the lower left corner of the client, where you type, there is a chat settings icon on the left; click it and switch on the system chat (the window of system messages) with a tick (do not switch on the damage, or the lines will run fast and you will not see whether the spoil landed or not). Take the tick off "system" at the top, so that nothing gets written into the main chat.
So, when the system says that the spoil has landed successfully, half the job is done. We finish the mob off and look at it: if the mob has turned blue, it means we will get something from it; press the 
Sweeper skill (it too has to be on the bar next to spoil) and receive the additional item or items. Spoil is also handy in that you can get not one kind of item at a time but two or more (up to four).
If the spoil landed but the mob did not turn blue, it means that although the spoil did land, the chance was small and we will get nothing from this mob. For peace of mind you can press sweep, the mob will vanish, and we do not despair and move on to the next one. In general any item can be spoiled: it all depends on the chance of spoiling this item from the given mob and on the spoil rates of your server. And on your patience, of course.
On la2wiki the spoil of every monster is listed on its page, and the page of an item shows every monster it is spoiled from.
Why so many dwarf girls of different levels
As a rule every trader has a heap of spoilers of different levels, so as to log in the right one at any moment and spoil the item required. I, for example, keep dwarf girls of these levels: two of level 80 (one with the quest for Varka, Varka Silenos Village, the other for Ketra, Ketra Orc Outpost), a 76 (for Wall of Argos, Hot Springs and so on), a 70 for the farm (Beast Farm), and then one every 5 levels. By the way, spoil goes very well with the manor: all the seed levels are shared out among my dwarf girls.
This is done because you have to spoil by the level of the mob: if the mob is far above you in level, most likely you will not even be able to spoil it, the spoil simply will not land; and if the mob is far below you, the spoil lands but you will not spoil the items, you just get a blank. So it is best for the mob to be within 3 levels of our spoiler girl. It does not always work out that way: if you go to the FoG (Forge of the Gods), the mobs there are a good 5 levels higher, so at level 80 we enchant the spoil skill without fail, or you will wear yourself out sitting on your behind regenerating mana.
Where to begin
The spoiler girl always goes first on a new server, since developing from scratch is rather hard; for me the easiest way is to spoil scarce resources on low-level mobs (and do some manor on the side), so we boldly go and register a dwarf girl. Then we start developing little by little: Levelling a spoiler: from the first mobs to level 60. The class quests of a spoiler: the first is Path to become a Scavenger (Scavenger), the second leads to Bounty Hunter, the third is Saga of the Fortune Seeker.
And yes, many do not use it, but the spoiler has crafting too. It is only of the first level, of course, but some resources can be crafted: those whose crafting level is one. These are 
Braided Hemp, 
Coarse Bone Powder, 
Cokes, 
Leather and 
Steel. Feel free to learn these recipes on the spoiler and, between runs after mobs, re-craft tons of basic resources.
Lately I am often asked where and what to spoil at level XX (put any number from 10 to 80 instead of XX), which resources to spoil and how to earn. I myself am no fan of narrowly specialised spoil: whatever is needed at the moment is what I spoil.