Last time, I talked about how important it is to make sure that the publisher or agent you're targeting with a manuscript actually accepts the genre, category, age level, etc. of the book you want to send them. That information should be easily available on the company or agency website.
Another piece of the submissions puzzle is to make sure you're sending only what the publisher or agency is willing to see. And that's also information which should be readily available in their submissions guidelines.
You've written your book, revised it, and now it's ready to send in. You can't wait for everyone to see it. The thing is, many publishers and agents will not look at the entire manuscript right off the bat. And yet the publishers and agents who don't want to see a complete manuscript first thing, often receive complete manuscripts from authors who haven't read the guidelines.
As you're preparing to submit your book, look carefully at what the company/agency wants you to send them. Some will ask for only a query letter. There are places where you can learn to write queries and have your query critiqued--including this blog--so take advantage of those resources if you aren't sure how to do a query. Under no circumstances should you say to yourself, "I don't know how to write a query, so I'll just send them the whole thing and hope for the best." At "best", your manuscript may be discarded.
(Not that anyone reading this would do that, right?)
Depending on the size of the company or agency, they may have a huge number of submissions coming into their email or mailbox. They don't have time to read every complete manuscript that every author who wants to work with them has written, so they use queries or "partials" (a set number of pages or chapters of your manuscript) to weed out the stories they aren't interested in from the ones they are. To some authors, it seems unfair to have to do "extra work" to get their manuscript in front of an editor or agent (and yes, I have seen it put that way), but it can be a time-saver for both the editor/agent and the author.
Before sending out your submission, be sure of whether the company or agent accepts full manuscripts, only queries, or queries plus partials. Do they require a cover letter as well? Do they accept submissions via email, or only through postal mail?
All of these questions should be readily answered in the submissions guidelines. If they aren't, locate a contact email for the agent or company and ask for clarification. And if you aren't able to find out the information, you may want to cross them off your list.