I know this has been probably addressed a thousand and one times, but I'm a lazy git and can't be bothered to search through all the forums so I thought I'd just ask again. lol.

Is there actually a difference between a query and a covering letter? If so, what sort of things do you need to mention that you don't mention in a covering letter. From what I understand, with a query you don't send any material along (like sample chapters), but does that mean you're ASKING them to look at your work?

A Query letter is what you send to US Agents, and you're right, you don't send any sample writing nor a synopsis.
They are like covering letters, but with the addition of a paragraph summarising your novel and ending in a hook - this is basically a sales pitch for your novel, and comparing it to similar novels in the market.
You need to make it interesting enough for them to want to call in a full or partial manuscript and a synopsis. They are similar to pitch leters that Agents send to Editors to 'sell' the mss.

You can find an example of a pitch letter on Synopsis Examples in the S&O Group. Also check out Nathan Bransford's blog where down the right hand side you will see 'Anatony of a Good Query Letter:
http://nathanbransford.blogspot.com/

- NaomiM

Wot Naomi said, though it also seems standard (says Miss Snark, as wall as Our Nathe) to send a page or three of your writing, along with the query, just to give the flavour.

Emma

Some agents I've been looking at do actually ask for some sample writings as well as a Query. I've started looking at Agent bios and see what they say in terms of submissions. That helps a great deal. If they just ask for the query, I just send them the query.