VHere's a mini guide on items! In order from worst to best: normal->enchanted->artifact->fabled->legendary->godly. Selling price also grows the better your item is (actually selling stuff is pretty useless after a bit because shops are so cheap). As you clear a stage, it will give you a part of a "set" from that area. It appears to give stuff randomly, so it may take awhile if you plan on getting a specific set/item/stat/grade.
 "Zombie Knight"){kind=icon}
 "Kingdom Rush"){kind=icon}