titles and forms of address

The British peerage, in order of precedence is:

duke/duchess: the Duke/Duchess of Somewhere, both addressed as Your Grace.
marquess/marchioness: the Marquess/Marchioness of Somewhere, addressed as Lord/Lady Somewhere. Note that sometimes the French form Marquis is used (though never the feminine French title of Marquise). Marquess is an older and purely English form.
earl/countess: the Earl/Countess [of] Titlename, addressed as Lord/Lady Titlename
viscount/viscountess: the Viscount/Viscountess [of] Titlename, addressed as Lord/Lady Titlename.
baron/baroness: Baron/Baroness Titlename, addressed as Lord/Lady Titlename.

The titles of duke and marquess are almost invariably territorial, eg Duke of Devonshire, Marquess of Salisbury, etc. The titles of earl, viscount, and baron are most often associated with a territory, eg Earl of Pembroke, but can also be based on a family name, in which case the "of" is dropped, eg Earl Spencer. A baron’s wife is not typically titled a baroness, though she is addressed as Lady Titlename. Only a woman who is a baroness in her own right uses that title.

The next two ranks are not peers, ie they do not sit in the House of Lords:

baronet: addressed as Sir Firstname, his wife as Lady Surname.
knight: addressed as Sir Firstname, his wife as Lady Surname; a knighted female is addressed as Dame Firstname, her husband as Mr. Surname, ie he does not share the distinction of his wife.

Whereas a baronet title is hereditary, a knighthood is not inherited.

For details on each rank as well as correct forms of address, these sites are recommended: http://www.debretts.com/forms-of-address/titles.aspx
http://laura.chinet.com//html/titles02.html

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