A query is a brief, interesting introduction to your novel. The purpose of a query is to entice the agent/editor to ask you for more. Queries are also useful for hooking potential beta readers. Queries are not to be confused with "back cover blurbs" or synopses. Queries are short -- 250 words is pushing it. 200 is better. And queries have a lot of work to do.
Agents and editors are a very particular audience. They know the genre -- probably better than you do. They've read a lot, and they've pretty much seen it all. They do not have time to waste. So let's not waste time either. Here's what you need to know about your story:
The main plot
Not the story. "A girl masters her magical powers" is the story. "Kate accepts the duties of healer to the prince during his secret mission across the dangerous mountains" is a plot. During the mission, she begins to master her magical powers -- a plot is the specific process by which a story happened. There are a million ways for a girl to master her magical powers. My story uses this one, and that's what the query needs to communicate.
The main character
This character is the focus of the main plot and the query. But I have an ensemble cast and they're all important. Sometimes two characters can fit into a query, but never three. You barely have time to tell your reader about the main character and the main plot -- everything else potentially adds confusion. Editors and agents don't have time to sort it things out. They'll just hit delete and read the next query.
In your query, you need to tell the reader:
This is an INTERESTING CHARACTER with a PROBLEM that he or she is DOING SOMETHING ABOUT despite the danger.
If you can't pin down why your character is interesting, what the (one) serious problem is, or what clear and reasonable things she's doing to fix it... you may have structural problems in the story (and I find writing a query useful for pointing that out to myself.)
So much to explain, so little time
There are going to be some things you absolutely must explain so that the above facts make some kind of sense. To go back to my plot statement -- Kate accepts the duties of healer to the prince during his secret mission across the dangerous mountains -- I have to pick out the most important things to spend words on. Who's Kate? What are a healer's duties? Who's the prince? Why a secret mission? Why the dangerous mountains?
Which of those questions is the most important? Which needs the most explaining? IMO: the secret mission. We also need a little explanation of why Kate is an interesting and sympathetic character. And if I have words left, I'll spend them on explaining the danger in the mountains.
Why nothing on the prince? He's a prince. People have a pre-packaged concept of what princes are and what they do, thanks to popular culture. And no, it's not entirely accurate to the prince in my story but it's close enough for a query. If a reader can't glean story potential from a teenage girl and a prince together on a dangerous journey, I can't help them in 200 words.
Canned phrases = deadly
In some places, "back cover blurbs" are used as examples of writing queries. Bear in mind, though, that back cover blurbs are written for a very broad audience -- anybody who might pick up the book. It needs to get its ideas across quickly to people who may not be familiar with the genre. To do this, many blurbs fall back on canned phrases that everybody recognizes. "She only wanted a normal life." "He struggles to overcome his past." "Her world turns upside-down."
Agents and editors are a very specific audience of experienced, jaded readers. Canned phrases are always vague, and vague does not tell the agent/editor why they should not hit delete and read the next query. Everyone's heard the joking movie trailer line: "In a world where..." Ever noticed that movie trailers don't use that phrase anymore? They know it's an over-used, canned phrase and it's deadly.
Queries are never right the first time. They are rarely right by the fifth draft, for that matter. This is normal. Post your queries here for feedback and keep revising.