A brief intro...
The Dresden Files RPG is based on a series of urban fantasy novels of the same name. The setting is a world much like our own but with various fantasy elements added. Unlike many urban fantasy novels the Dresden ones are plot, not porn, driven.
The vast majority of people in this world are normal humans. They don't believe in magic and when they see it they rationalise it as something else. Post a film of a battle with a werewolf on youtube and people will comment on how lame the special effects are. Show them magic and within a week or two and they will have convinced themselves that they were conned. There is no supernatural force behind this, merely humanity's need to see the world in rational terms. In theory the mortal authorities could be told about the supernatural, but this would spell disaster for the supernatural characters as humanity declares war on them. Getting normal humans involved in the supernatural struggles is often called "going nuclear" because of the risks involved. That said, in most cities there are a handful of cops (or other people) who know about the supernatural and deal with how it affects the rest of the world.
In this world there are full-blown wizards, sorcerers, and people with minor talents. Some, otherwise normal, people have faith to the point that they have Faith driven powers. Along side them are at least three different kinds of vampires (a fourth has been mention but not defined), ghouls, werewolves, and countless other monsters.
Note: all "full powered" wizards are part of the White Council. This council has declared that there are seven laws of magic and enforces them on all humans that use magic.
There is also a magical world that exists along with this one - called the Nevernever. The Nevernever can be entered by wizards, some sorcerers and minor talents, some monsters, and occasionally by accident. The real world doesn't map directly to the Nevernever - unless you experiment you don't know what is on the other side of the barrier. In this other world there are fairy kingdoms, spirits, monsters, demons, goblins - if you can imagine it then odds are it exists there.
While most of the fairies live in the Nevernever, some minor ones live in the real world. Fairies are basically broken into Summer Court, Winter Court, and "Other" (called Wyldfae) and the most powerful of them can't harm you unless you've entered into a deal with someone from fairy.
Virtually everything that lives in the Nevernever can interbreed with humans. The majority of these offspring are fairy half-breeds (called Changelings) who at some time in their life will have to make a choice - live as a normal person (giving up all their fairy powers) or become a full fairy (giving up their humanity and the bulk of their freewill). Others are descendant from other Things and are call Scions.
One of the reasons that humanity can remain ignorant of the various things out there is that most of the powerful groups of creatures have agreed to follow something called "The Unseelie Accords". These basically say that each group has total say over its own membership and define how they can peacefully co-exist. Most of it boils down to ignoring each other and letting every group get on with what they do. There are rules in there for duels, self defense, paying a wergild to settle transgressions, and what is a legitimate reason for war. An important thing to remember is that there is no spirit of the rules, only the letter, and some beings will twist the accord anyway they can. Both fairies courts, the White Council (a.k.a. the wizards), three vampire groups, and several Dragons have all signed on to the Accords.
As an example of how the Accords work, in the early Dresden books the hero (Harry Dresden, a wizard of the White Council) knows that there is a group of vampires in his city but cannot do anything about it unless they cross certain lines. He knows that he could battle them but if he attacked them outside of what is allowed by the Accords he would risk starting a war between the vampires and the White Council.
Oddly enough, most of the 'refined' monster types (vampires, fairies, etc) respect what could be called "Old World Manners". If you are an acknowledged guest they won't attack you and if they give you their word then they will keep it. Often you can avoid a fight with such a creature as long as you are polite and don't make a point of standing in their way.
Into this mix comes your PCs. You get to pick your template and powers and then join in the fun. Odds are you won't want normal people to know about any powers you have, but there are enough weird things out there that you won't feel alone. While some gaming groups might have a different focus, the setting assumes that you will be one of the people trying to keep the world safe from dark forces while keeping humanity ignorant of the things that go bump in the night.
A bit about the FATE system:
The FATE system isn't about dice rolls. While you make your characters (it's a group effort) you assign them Aspects. Aspects are hard to describe - they are kind of like Metaskills. They define who your character is and if an aspect says you can do something then odds are you can do it without dice rolls. If dice rolls are need you can 'tag' an aspect for a +2 bonus or a reroll. You can tag multiple aspects for every roll and you decide whether or not to tag them after you roll (and after rerolls), but every aspect can only be tagged once per roll. The first time you tag it you tag it for free, but after that you need to pay a FATE point to tag them.
FATE points drive the game. You start with so many of them (the more powers and stunts you buy the fewer you start with) and gain others during play. FATE points can be spent in many ways (gain a +1 for a roll, tag an Aspect for a +2, declare that you have something that you normally wouldn't have, etc) and are gained in various ways.
During play you can make Declarations - either through using a skill or paying a FATE point - which are things that add to the game. The GM has final say over what you can declare but generally speaking as long as it's interesting and adds to the story the GM will allow it.
Skills all have a word associated with them - something like Average, Good, Epic - and they basically describe how good you are when you use that skill. For example, if you have Investigation at Great you're at the level of the people on CSI.
During play you can (with GM permission) gain more powers and stunts, but this will cost you refresh points (which means starting play with fewer FATE points). But beware - refresh points are tied to the concept of freewill and should you ever reach zero refresh your character entered NPCdom.