The quote is a letter from a service or goods provider, which is a prima facie statement of costs for those products.
The quote should be itemized and cost specific. The provider is giving a current set of prices.
Note: prices may be qualified by caveats on time frames and availability of stock, etc. These details are extremely important, because the supplier isn't bound by their quote if those conditions don't apply.
Your letter in response can take several forms. You can acknowledge or question quotes, depending on circumstances.
You, as a purchaser, are not bound by a quote of you don't make any commitments to purchase. If you're not ready to make a decision, make it clear you're still shopping around.
The example below is a mixed bag of both acknowledgement and queries, covering both situations.
Dear (use full title of the addressee)
Re: Quote for installation of air conditioning at 123 ABCD Street Wherever
We acknowledge receipt of your quote dated (insert date).
We would like to raise several points for clarification:
We are currently considering various suppliers for this contract. We would appreciate your response to these points prior to 30 April 2009. If no response is received it will be assumed you no longer wish to tender for this installation.
If you wish to discuss any of the matters raised, please contact me on the above phone number or email.