I'm doing a whole lot of research into Medieval times for a book I'm writing, and many of the books I'm reading mention Knight Bachelor or Knight Banneret. I'm very unsure what the sognificant difference is. I know that knight-errant seems to be the closest to "hedge knights" in A Song of Ice and Fire, and that the young William Marshall is a good example of a knight errant, but I can't find any explanation for bachelor and banneret that have helped me so far. I would greatly appreciate some help in the matter. :)
This answer is about Knights Banneret in England during the hundred years war.
During the 14th century in England knights were allowed to fly a pennants (flags that came to a long tapered point) but were not allowed to fly a regular rectangular banner unless they were awarded the rank of knight banneret. It was an award given to knights who were leaders of men, not (at least typically) just for individual prowess or bravery. It was often given at a battle. Typically during the ceremony they would cut the tail from the knight's pennant, transforming it into a rectangular banner.
For example Hugh de Vere was a knight banneret. His retinue included 7 knights, 20 mounted archers and 24 squires.
This information comes from William Caffero's book "John Hawkwood: An English Mercenary in Fourteenth-Century Italy"