When you submit your book for consideration you can contact agents or publishers, I have looked into both. When considering an agent you will almost always send a query letter while publishers may have different needs. Regardless of which you pick it is important to do your homework in order to send exactly what they are looking for. Most agents and even a majority of publishers do not want to receive your entire manuscript right away. Their websites often times will even say that unsolicited manuscripts will be discarded.
Not only will it get your work thrown away but it makes you appear extremely unprofessional. So once you pick out your agents and publishers it is vital to not only get their addresses, email addresses, names and agencies but also to know what they will accept. Starting with the query letter for agents I came across dozens that only wanted the letter. In facts about seventy percent of the agents I queried desired a query letter as an email and nothing further unless I was contacted by them.
Because they are busy and constantly being bombarded with queries they will tell you that they may not respond to everyone and while some do give an expected response time there were a handful that I simply never head from again. Once you have the letter printed and the envelope stamped you may include a few pages if the guidelines call for them. In the case of an email follow the given instructions. It is easy to just add an attachment but because of all the viruses online many will ask for the first chapter or so many pages pasted into the body of the email itself.
Publishers can be even more specific. Not only will they tell you what they want in a query letter, how much of the manuscript to include or not include but also they will tell you up front exactly what kinds of books they publish and then you can figure out if they may be a good match. They will give brief examples of the types of books found in their lines and I have seen a number that are giving word count ranges because they may or may not consider novellas or epics depending on the branding of their company.
Make sure when you are preparing to submit that you read all of the information carefully. If you are like me and submitting to a number of publishers and agents simultaneously it is also a good idea to make sure you keep the information organized so you don't accidentally send a single letter to an agent looking for the first three chapters or send the entire manuscript before you were asked by another. Knowing who wants what and keeping track of who you have contacted will make the road slightly smoother and you travel your publishing journey.