Much of the obsession over titles and orders of aristocracy are a modern invention, or at least based on a narrow reading of late medieval history. Through most of the period, it was rather more chaotic than that, with a lord's title (lord, earl, count, viscount, duke, what have you) not necessarily reflecting his power or standing. Barons wrested with earls and counts, and were not seen as somehow defying their social betters; viscounts defied dukes; knights in cases developed wealth and power to rival titled aristocrats.

To sum up: a knight is an aristocrat, and may be a lord; a lord is an aristocrat, and almost always a knight. A knight who acquires land through marriage, inheritance, or service may well become a lord.

A great example of the confusing nature of this is in the life of William Marshal. Marshal, the younger son of a relatively minor Anglo-Norman lord, did not own an acre of land until he was well into his forties. Prior to that, he had served several nobles as a household knight, culminating in his service as a key adviser and lieutenant to Henry II. When Henry's son Richard came to the throne, he arranged a marriage with a powerful heiress for the Marshal, and he became overnight one of the most powerful landholders in England. He served as a key subordinate to Richard's successor, John, arguably being the power behind the throne. William ended his life as one of the most powerful landholders in the kingdom of England and the regent during Henry III's youth.