What precisely consists of a book proposal will vary according to the publisher. Most companies state clearly on their website what would-be authors should supply to them. You'll find that some of the largest publishers actually say they won't even consider submissions that don't arrive via agents. The rest of them will usually ask for a single page proposal or a covering letter, a synopsis and between one and three sample chapters. Accompanying this with a large envelope and sufficient postage for the sample chapters to be returned is actually a pointless thing to do because you should never re-use those samples anyway. It's far better to enclose a postcard (with a stamp on it) addressed to you that the publisher can acknowledge receipt of your submission. If the proposal is rejected the sample chapters can then be thrown away or recycled by the publisher. Make sure your e-mail address is included in your covering letter so that the publisher can send you a reply without incurring any costs.
Your sample chapters must be cleanly printed on fresh paper. If your toner or ink is running low make sure you replace it before printing anything. Publishers don't want to read weak and smeary text.
That's straightforward enough, but it doesn't tell the whole story. The purpose of a proposal is to give enough information to an editor about your manuscript for them to make a judgement as to whether they would like to read more. But there are other ways to tantalise a publisher, as we'll see later.