In the UK it's called a covering letter, in the US it's called a Query letter, and although there are similarities they are different animals.
Both include your contact details, details about the mss - title, genre, word count - and a brief bio. and mention of any writing credits (comp. wins/placements, publications), but there the similarity ends.
In a covering letter include an overview of the novel, summarie to a paragraph, which introduces the mc, the setting, the chief antagonist/conflict, which shows it fits that genre.
In a Query you're Pitching the novel in the hope that the agent will ask to see a 'partial' or 'full' mss, plus synopsis.
It's not necessary to pitch your novel in a covering letter because you're already including a 'partial' (1st 3 chapters and synopsis), so it's best to avoid having anything in a covering letter that'll STOP them from reading on. Keep it simple and buinesslike.
- NaomiM
<Added>More detail here
http://www.writewords.org.uk/archive/22925.asp plus advice on writing a 'targetted covering letter'.