Attempting to figure out the line of Peerage can be a bit daunting. So I am going to do my best to create a bit of a guide. Bare with me!
Peers have a family name as well as a title; Georgiana Cavendish, Duchess of Devonshire. Peers have more than one title and go by their highest title. Their eldest son will take his father's other titles when he is born and all other males in the family are considered commoners. When the father dies the eldest son inherits the highest title and his wife will now have the title of Dowager in front of their title; Dowager Princess of Wales. If no male heirs are to be found in the correct bloodline the title will become extinct, such as the Dukedom of Portland which expired in 1990. If a daughter of a peer marries a commoner the title Honourable will be added before their name, such as The Honourable Mary Graham.
The peerage has a hierarchy which is as such:
DUKE and DUCHESS
Address: "Your Grace" (ie: Her Grace the Duchess of Devonshire)
Style: The Most Noble or His/Her Grace the Duke/Duchess
Rules: a duchy
Coronet: Eight strawberry leaves, five in two-dimension
The Duke of Cornwall is always the eldest son of the monarch, the monarch is the Duke/Duchess of Lancaster, the Duke of Norfolk is the Premiere Duke of England, the Duke of York is usually the second eldest son of the monarch (the Prince of Wales is the next in line for the throne).
MARQUESS, MARQUIS and MARCHIONESS, MARQUISE
Address: "Lord" or "Lady"
Style: The Most Honourable Marquess, Marquis/Marchioness, Marquise
Coronet: Three visible strawberry leaves, two visible silver balls (or pearls)
EARL and COUNTESS
Address: "Lord" and "Lady" (ie: My Lord Essex, Earl of Essex)
Style: The Right Honourable Earl/Countess
Coronet: Four visible strawberry leaves, five visible silver balls (or pearls)
VISCOUNT and VISCOUNTESS
Address: "Lord" and "Lady"
Style: The Right Honourable Viscount/Viscountess
Rules: a Viscounty, viscountship, viscountcy
Coronet: Sixteen visible silver balls
BARON and BARONESS
Address: "Lord" and "Lady" unless the woman is a Baroness in her own right then she is "Baroness"
Style: The Right Honourable Baron/Baroness
Rules: a Barony
Coronet: Six silver balls or pearls
BARONET and BARONETESS (very rare) and KNIGHTS
Address: "Sir" and "Dame"
...are not considered peers. The title of Baronet could be inherited but not the title of Knight.