Skills in a field (on casting and how many spells you know)
Novice - Starting Learning (1-2 year)
Adept - OWL Level (3-5 year)
Apprentice - NEWT Level (6-7 year)
Journeyman - Hogwarts Graduate
Master - Master Duellers, those who focus on getting better and learning more, etc
Grandmaster - Simply put Masters who have lived long enough without getting complacent.
Knowledge (Theoretical mostly but inventions as well)
Apprentice - NEWT Hogwarts age
Journeyman - Pursuing mastery
Master - Completed mastery in a field
Grand Master - Mastery in many subjects or deep knowledge in many. Generally 3. (Dumbledore, Unspeakables etc.)
Wizard Power
Squib
Below Average
Average wizard
Above average (Novel Cedric Diggory, Viktor Krum)
Adept (Members of nobility of this book and masters in a field of magic, Hogwarts professor other than heads of houses, aurors like Tonks etc.)
Elite (James Potter, Sirius Black, Lily Potter, Moody, those who with proper planning can hold on mages for a while)
Mage (powerful wizards who could wipe out small countries without using destructive spells like fiendfyre)
Archmage (Merlin level, Dumbledore, Voldemort, Grindelwald, master of many fields of magic, atleast five)