Any ability which a character possesses that is not an inherent trait falls into the category of a skill. Skills can be anything from knowing how to aim a bow to being an expert in a scientific field. They can be magical spells or psionic powers. A skill can be a profession which the character practices, a knowledge of a geographic area, or even something as broad as being an Native American.
Skills can cover very wide areas of knowledge or they can be minutely specific. Exactly how individual skills are defined and what they "cover" is up to the imagination and common sense of the actors and the ruling of the Storyteller. It is far more important that a skill describe a character well than be exactly clear as to what they allow them to do.
For example, if your character were a private investigator, you could choose to list two dozen specific skills describing his strengths and weaknesses or you could simply put "very experienced private investigator." You should include enough detail to represent the image you have of your character without bogging yourself down with minutia.
Again, the idea behind skills is that they should accurately represent how people perceive your character and what they can do. If your character honestly knows six different ways of cooking an omelette then feel free to list them all singly; it's up to you to decide what's important toward understanding your role.
When describing a skill, you can use whatever adjectives fit your vision. Following are some examples:
Professional UFO investigator. (D10)
By now you are probably looking at those dice and wondering where they are coming from. Read on!