Nobility refers to peers and their families. The peers are barons and above, and sit by right in the House of Lords.
Gentry refers to anyone gentle but untitled, usually descended from nobility.
Knights are not noble. They are knightly. Knights and peers' sons may sit, by election or appointment, in the House of Commons. An ordinary, undifferentiated knight is a Knight Bachelor . Knight Banneret is an honour conferred on a man who distinguished himself on the battlefield in front of his monarch.