In regular temperaments like Pythagorean tuning and meantone temperament all the fifths are the same size, except one (typically Eb-G#) which is badly out of tune (a "wolf"). This means that as long as you stay with keys that don't involve the wolf fifth, eveything is fine, but the remote keys are unusable.
Well temperaments have fifths of different sizes (hence the name "irregular temperament"), usually smaller ones round the home keys to get sweet thirds, and bigger ones in the distant keys to make it all add up.
In a well temperament you can modulate through all the keys (hence the name circulating temperament), but all the keys have slightly different tuning, hence distinct characters.
Well temperaments evolved in the baroque period, were current in the classical period, and survived into the late nineteenth centuary. Bach's Well Tempered Clavier was written for a well tempered instrument (not for equal temperament as is commonly stated)
There are many well temperament schemes, some nearer meantone temperament, others nearer equal temperament: