This is a comparison of the three different levels of YouTube quality.
Standard Quality (SQ), there is very little colour, it is both too sharp and too blurry and it is composed of very large squares. Not good quality.
High Quality (HQ), an almost perfect quality, although some areas are still too sharp. Text is readable but there are some misplaced pixels.
High Definition (HD), this is the best quality on YouTube. Extremely similar to HQ. HD is available by clicking the link at the top of the description.
SQ supposedly loads quicker, try to watch in HD or watch in HQ if you want the best of both