I agree with Waylander that they are very different creatures, but I disagree that the query is the back of the book blurb - IMO, queries are different from both the one page synopsis and the back of the book blurb.
I agree with Waylander that the one page synopsis is the bones the of the story, including the ending - ie, all of the absolutely essential plot events that move the story from start to finish. And the most effective synopsis in my exprience focus on the cause and/or effect of those absolutely essential plot events.
The Back of the Book blurb is designed to hook the average reader, it rarely gives away anything beyond the beginning of the book/book's set up, and is intended to entice the average reader to buy the book.
Because the query is intended to entice the agent to scroll down or flip to pages, it also is intended to entice. But because the agent is more savy than the average reader, and looking at the query with a more critical and dissecting eye, it is usually approached a bit differently than the back of the book blurb. Now, I say usually because queries vary widely and some that are written as back cover blurbs do in fact lead to partial/full requests - which is the sole purpose of the query. The reality of queries is that sometimes even bad queries achieve partial or full requests if the concept for the book or something else in the query captures the agent's interest and she or he looks at the pages. But in general, from my experience, the query needs to give more away than the book cover blurb and needs to be more focused on the primary conlfict in the book, as opposed to the book's set up.
There is much more info on queries and synopsis in the Query letter Hell sub forum in the Share Your Work forum.
ETA: And yes, queryshark is very, very helpful as well. And if you google other agents, you will often find otehr agents who have posted queries for critique on their blogs or judged queries as parts of online contests, and so can find otehr agents' views of queries, as well.
~suki
