Easy puzzles suit first-timers, returning players and anyone who wants a relaxing few minutes. Every puzzle has exactly one solution and can be finished with singles alone, so you never need to guess.
Both can be solved with singles only. Medium has more empty cells (45-50), so you need more hidden singles and longer scans.
Yes, if they know the numbers 1 to 9 and the rule that a number cannot repeat in a row, column or box. Easy is a good place to start.