The single scull is a class of rowing shell. It is arguably the most extreme form of rowing in existence; there are boats with more rowers, more oars, or more speed, but the single scull is rowing brought down to it's essence: one man, one seat, two sculls. The sculls are the oars the rower holds in his hands, one for each side; enabling the possibility of a straight course. It is the slowest way to race, requires the toughest mindset but gives the greatest personal victories in the sport.
A single scull also makes it possible to cruise along at your own leisurely pace. The world is often prettier or more interesting from the water, and strolling along in your single scull gives one the perfect opportunity to admire the view. This is for me one of the major advantages of the single scull. Larger boats do not offer this individualising opportunity, crewmates expect an amount of effort out of all rowers, killing your opportunities to look around. When I want to train for my fitness it is best to row in a crewed boat, teammates keep me sharp.
One of the major disadvantages is that a single scull is unstable as hell, and very prone to flipping over. At 60cm wide and 8m long the shell is very narrow and is likely to turn over on it's own accord. Balance in the boat ('set' in rower's jargon) is kept with the sculls. When lying still the only decent way to keep balance is with your blades on the water, without those it's as difficult as walking a slack rope.
This means that the unexperienced sculler is very likely to get wet on his first attempt, which is no shame, and this baptism will take away the fear of flipping for future rows.