A "Jack-of-all-trades" is someone who is good at doing many different things and is used in a positive way. An example would be someone who can fix cars, build a house, and cook Italian food. A "know-it-all" is used in a negative way to describe someone who speaks as if they know everything. Example: A person who says what they know on every subject, even when no one asks them. Is that more clear?

Here is a well known proverb: A jack of all trades, but a master of none