In most cases Assembled cylinder heads are pre Assembled with valves, valve springs, locks, retainers, and seals. They are a great choice for taking the guess-work out of the equation compared to starting with bare un-assembled heads.
Bare cylinder heads are just that, "BARE". They do not typically include valves, valve springs, locks, retainers, and seals. They are a great for building your cylinder heads to your exact specifications. They are normally sold individually allowing you to purchase a single replacement if needed.