Characters have broard areas of knowledge and ability, and within those areas they have specific skills which apply to their day to day lives. The broad areas are called "Proficiencies" and the stricter, smaller areas are called "Skills". A character may have many Skills in any given Proficiency, and in some circumstances they may have skills even though they have no proficiency that relates to it. Proficiencies are rated, like attributes, as a value from one to five (or above for exceptional individuals). Skills do not have values, they simply represent areas of a expertise in their Proficiency that a character has.
To give an example:
Mechanics is a Proficiency. There are many Skills within that, such as: Cars, Trucks, Chariot/Carriage, Engines (petrol), Engines (Diesel), Engines (Thaumaturgical), Firearms, Hybrid weapons, Pumps, Boat Systems, Ship Systems, Fixed-wing Aircraft, Dirigable, Submersible, and a whole lot more besides..
This is by no means an exhaustive list however, and players should note that no skill list is EVER exhaustive or totally complete, regardless of what certain games designers or publishers may tell you ;^ )
So a typical Car Maintenance mechanic might have:
Mechanics - 3
Car,
Petrol Engines,
Diesel Engines
A really good, highly trained mechanic might have:
Mechanics - 4
Car,
Petrol Engines,
Diesel Engines,
Thaumaturgic Engines
In Car Entertainment Systems
Chariot/Carriage
When a character tries to use a skill, they will usually combine the most appropriate attribute with the rating they have in the proficiency that the skill they are using is in. In the case of our mechanic, he's usually adding his Mechanics Proficiency to his Mind attribute.
If a character is attempting to use a skill they do not have, there is a penalty to this number. Generally this is 3, but some skills are harder to guess at than others and some easier, so this might be different. For example, if our typical mechanic is trying to work on a vehicle with a Thaumaturgic engine, he will be effectively operating as if his Proficiency was 3 - 3, zero.
Bear in mind also that some skills bring with them certain other abilities which look like ther belong in other Proficiancies. For instance, a character with the Drive proficiancy and the Car Skill will know the basics of car maintenance; changing the oil and checking tyre pressure, but wouldn't be able to strip the engine beyond the spark-plugs or timing crystals.