In Westeros, knighthood belonged to the Faith of the Seven. Knights were always nobles, but not all nobles were knights. Simply put, any knight could make someone else a knight, whatever the reason.
A knight's social standing fell somewhere between that of a lord and a commoner.
Unlike nobles, knights had no land of their own (though they could swear fealty to a lord in exchange for land) and could not pass their title down by inheritance. (However, a knightly father or other knightly relative could knight a son, passing down the status, after which the son could continue swearing fealty to a lord in exchange for keeping the land.)
Even people of low birth could become knights. Once knighted, they were addressed as "Ser" before their names. (Westeros had many landless knights and wandering knights who served no lord.)
Even someone who had not grown up in the knightly tradition and did not worship the Faith of the Seven could be knighted for outstanding deeds, such as a brave soldier or a commoner who had rendered great service. Their social standing rose by a whole level as soon as they became a knight.
After being knighted, one could use the title "Ser," enter the nobility, and become eligible to bear a personal coat of arms, compete in a tourney, inherit land, have subjects, be chosen for the Kingsguard, take part in the assemblies of high nobility, and receive invitations to noble weddings. Knights also enjoyed better treatment as prisoners of war. In short, they had one foot in the noble order.
Lords had legitimate rights that knights did not. For instance, the protagonist was a petty lord. Once his land was granted to him, it no longer had any connection to its previous owner; it became land inherited by