They also provide an email address where, if you send an email with some epubs attached, they make them magically appear on your kindle.
I was pleasantly surprised by this last week, since I got a kindle and I thought it was going to be a hassle, but nope.
When I tried emailing an epub this way, Amazon messed up the conversion. I've had good luck converting pirated epubs to MOBI using an online converter so I haven't changed my practice.
I have an older Paperwhite; my GF has a newer one. I uploaded MOBIs to both last night (via USB) - it worked on hers, and they didn’t show on mine. I had to convert them to AZW3 and reupload for them to show in the listing.
(I’ve not checked, but it’s plausible my older model has more up-to-date firmware.)