There are five grades of the Peerage:

The highest rank in the Peerage was introduced into England in 1337 when King Edward III created his eldest son Duke of Comwall. The first non-royal dukedom was created in 1448. In Scotland, the first two dukedoms were created in 1398.

The second rank in the Peerage is derived from the Gernlan Markgraf, signifying the guardian of a March, or border territory. King Richard introduced it into England in 1385, and it was introduced into Scotland by King James VI in 1599.

The third rank in the Peerage, but the oldest. Its origins are traced back to Saxon times, when an Ealdornlan administered a shire or province for the king. The present title of Earl, derived from the Scandinavian Jarl, dates from the reign of Canute (1016-35).

The fourth rank in the Peerage is derived from the hereditary office of Vice-Comes, that is, the deputy of a Count. King Henry VI introduced it into England in 1440.

The Normans introduced the fifth and lowest rank of the Peerage into England. A Baron was tenant-in-chief of the king, holding his land directly from him. In Scotland, the equivalent title is Lord of Parliament, as the word 'Baron' relates to Feudal Barons.
Taken from Burke's Peerage and Gentry 106th edition. The order of Precedence begins with the Sovereign. The Peers, of which there are five grades, are detailed in the order of precedence below, beginning with the highest rank of Duke.