Structuring the synopsis

I have a novel which I am submitting to an agent. It is ready to go except for one thing - the dreaded synopsis. I am having a lot of trouble with the specifics of it. Is is an outline of the plot or an examination of the themes in the novel? Do you have to explain who all the characters are? I've been told to keep it one page and am struggling. Please help. Thank you so much.

The synopsis is a very important document, so it's worth working away till you get it right (or right enough). The synopsis will form the basis for the publisher's proposal to their Acquisitions meeting and also, later, for the blurb on the back on the book. It will also, in the first instance, tell an agent whether they want to read the rest of your manuscript. However, the synopsis is different to the pitch - the pitch is the one- or two-paragraph outline that tells the agent/publisher why they should snap up your novel. The synopsis is the the follow-up document to the pitch, giving detail about plot and characters.

The most effective synopsis I have seen told the story of the novel (through to the end) in one page, and gave minimal character explanation. The characters were mentioned as needed, when they popped up in the story, and this worked well. Themes were touched on in the context of the storyline, not as a separate note. I've never decided to read a full manuscript based on character explanations or descriptions of themes, but I have decided to read on based on a synopsis that told me where the story was going.