The following list of skills is not intended to be a comprehensive list. Rather, it is intended to provide descriptions of some of the more common skills, and give ideas for how other skills would work.
In particular, we have avoided almost all trade skills, as there are more trades than we have time or space to define. Sure, maybe we should have included carpentry, plumbing, masonry, electrical, electronics, smithing, medicine, dentistry, writing, cooking, farming, and so on. But then we'd also have to include weaving, sewing, cobbling, haberdashery, ranching, newsgathering, smelting, logging, refining, and all the other myriad skills which people may have, and we'd miss some for sure.
Many of those employed in so-called "white collar" jobs can be defined in terms of the skills included here, thanks to the broad Knowledge skill. "Blue collar" workers (without whom there would be no white collar workers) tend to have a much more varied range of skills, each of which may be dependent on different physical or mental attributes, which we cannot hope to define a single skill for.
Do not hesitate to define new skills when the need arises. The player and GM should sit down together and decide what the primary and secondary attributes should be, what skills (if any) will be related, what the default skill level will be, and how the skill will work in practice. Don't worry about getting something wrong; as long as it makes sense to you, that's the important thing.
The only word of caution we have is that you should try to keep things as simple as possible. Many times, we have come up with something that was just too complex, but after simplifying it, it often makes even more sense, and more accurately models the situation, than the complex version.
A difficulty rating (often just "difficulty") is a number assigned by the GM which indicates how difficult a particular task will be. The skill descriptions often assume a task of trivial difficulty. If the task is more difficult, a positive difficulty rating will be assigned. Negative difficulty ratings are rarely used. The difficulty rating is subtracted from the character's skill level. Not all skills may be appropriate for difficulty ratings (samples difficulties are given for many that are).
A modifier is a number, which may be assigned by the GM or a result of a previous action, which takes into account the circumstances surrounding the skill check. Unlike difficulty ratings, a positive modifier makes the task easier, while a negative modifier makes it harder. Modifiers are added to the character's skill level. Under the right (or wrong!) conditions, almost any skill may be subjected to modifiers.
Dirk is trying to pick a very secure lock without his normal tools. The GM might assign a difficulty rating of 30 because of the quality of the lock, and a modifier of -20 for the lack of tools. These two will combine to reduce Dirk's Lockpick skill from 63 to 13(=63-30+(-20)=63-30-20). If Dirk were to get his tools, the 30 difficulty rating would not change, but the -20 modifier would be eliminated giving him a 33(=63-30).