First you must know that an "imprint" is a division within a large publisher. Various imprints specialize in either different styles of book, or a particular editor's own aesthetic. Like, for example, Simon Pulse is almost always going to be highly commercial, edgy YA fare. Puffin is almost always going to be paperback reprints. Little Simon is always going to be board, pop-up, novelty and picture books for the PreK-K crowd. Arthur A Levine books might be anything from Picture Books to YA, but will almost always be special, literary books with something of a timeless feel.
You can get to know the types of books each imprint publishes by looking them up online of course (here's a list of all Penguin imprints with each one's history, and here's a list of Harper childrens imprints) - but to my mind, nothing beats spending a lot of time in the bookstore or library and keeping your eyes peeled for the publishers colophon (fancy publishing word for logo) on the spine. You will see, as you read more and more and pay attention to who publishes what, that most imprints really do have their own discernible style.
Here are some examples of children's and YA imprints, by publisher.*
Macmillan - includes FSG, St. Martins, Feiwel&Friends, Holt, Roaring Brook, Tor among others
Penguin - includes Dial, Dutton, Putnam, Viking, Razorbill, Puffin, among others
HarperCollins - includes HarperTeen, Katherine Tegan Books, Rayo, Balzer&Bray, among others
Random House - Knopf, Bantam/Delacorte/Dell, Schwartz&Wade, Wendy Lamb Books, RH Kids, among others
Simon & Schuster - Atheneum, McElderry, Little Simon, Beach Lane, Pulse, among others.
Scholastic - AAL, Scholastic Press, Orchard, among others.
Then there are places that I consider "one-and-done", where the editors seem to work together more and don't really have significant divisions, including Candlewick, Bloomsbury/Walker, Chronicle, Egmont, Hyperion, Little Brown, Flux, Sterling, Sourcebooks etc. They might have different lines for different types of books (like Sourcebooks Fire YA line, Little Brown's Poppy line or Candlewick Sparks early readers) but those don't quite constitute their own departments with their own dedicated staff.Within each imprint there may be anywhere from one to a couple to a conference-room-ful of editors. Every one of those editors have different tastes and specialties. There may be only one editor at an imprint who likes fantasy, or nonfiction, or whatever it is, or there may be several. We want to target the book not just generally ("This seems like a Knopfy kind of book") but also specifically (Who at Knopf is looking for a book like this? Whose taste would this suit? Who has something too similar in tone already?). That is why it is so important for an agent to know not only the style of each imprint, but the tastes and preferences of as many editors within each imprint as possible.
Some publishers allow simultaneous submissions to multiple imprints, some do not. But actually, even when it is theoretically allowed, it is not a practice I am personally fond of. Unless there is some pressing reason to submit a project to two imprints at once, they both won't be able to offer, so I feel like that can just cause bad blood and political wrangling that is not worth it.
Still, even if one can only submit to one editor at an imprint at one house at a time, that is still a lot of imprints, right?? RIGHT??? Welllll... not exactly. Because we also have multiple PROJECTS going out at once. I personally try hard not to send two projects to the same editor at the same time because I don't want to "burn them out" (though frankly sometimes it is just unavoidable). This might be my own personal issue - I feel like it is really hard for anyone to seriously consider two projects at once. What are the chances that they are really going to buy both of them? My projects have enough competition from the outside world, thanks, I don't need to create my own competition.
I also hate to send the exact same kind of project to an editor in quick succession. I feel like, if they reject a paranormal romance from me, and then the following week I come back to them with another paranormal romance, that is likely to elicit a big sigh.
And, if you are talking about a non-debut author, they might have existing relationships with certain houses or editors. Perhaps their editor moved to a new publisher... Or perhaps their publisher declined to make an offer on a certain manuscript... these scenarios can certainly affect the list.
As I am sure you can see, that means that the available-editor list is getting smaller and smaller. Let's say I have two fun fantasy projects going on at the same time, and I don't want to repeat any editors. One of them might go out to 8 editors and sell right away (yay!) but the other one goes out to a different 8 and doesn't. I end up going a second round of editors, or even a third. Eventually I am going to run of editors. And if during that time another big fantasy project comes along, but I still have project B out with a bunch of folks, and I just sent project A out not so long ago... Yeah. Gets a bit intricate.
Now if you'll excuse me, I have to get back to my list-making hidey-hole!
*ETA: this list of publishers & imprints is for example only, and is by no means exhaustive. And I am referring only to USA children's book publishers.