Skilled or skilful?

A person can be skilled or skillful, but a thing can only be skillful. This is sometimes an important distinction: "skillful work" is work done with skill; "skilled work" is work calling for a skilled (or skillful) person to execute it. Also, only skillful has an adverbial form, so for consistency one may want to say "skillful" if one later intends to say "skillfully."

Thanks very much Ruricolist - that's very useful.

Is the difference between these two terms the same as for skillful and skilled above? Thanks.

sorry, forgot to put the words: "efficient" and "effective"

Efficient includes the concept of economy of effort and time--thus the job is done as quickly and inexpensively as possible. Effective includes the concept of usefulness and appropriateness--thus the job is done with an eye to the end result being beneficial. IMHO anyway.

How do we differentiate between skilful person and knowledgeable person.

@calvin mpilo - A skilful person is someone who is good at doing something, whereas a knowledgeable person is someone who knows a lot about something. Someone may be very knowledgeable about plants, for example, but not be very skilful at growing them, whereas their green-fingered neighbour might be very skilful at growing plants, although not having a great theoretical knowledge about them.

An art historian might be very knowledgeable about art but hopeless at painting; a self-taught watercolourist, on the other hand, might be a very skilful artist without ever having read a book on the subject.

The skilful person no doubt learns a lot through practice and so becomes knowledgeable about the practice of their skill (although not necessarily the theory), but there's no guarantee that a knowledgeable person is going to become similarly skilful at anything.