Aspect ratio is the relationship between an image's width compared to its height. I almost exclusively use an aspect ratio of 3:2. When you order prints, regadless of wether you use this website to do so or whether you venture out and have them printed at one of the instant print stations or order them through another website, make sure you always use the preview option when you order prints sized other than 4 by 6. The 3:2 aspect ratio makes for a perfect uncropped 4 by 6 print. An 8 by 10 print on the other hand, has an aspect ratio of 5:4, which means that you may lose some important parts of a picture when it is cropped to this aspect ratio.
When you order prints of images I have created with an aspect ratio other than 3:2, again, use the preview option, so you don't lose parts of the photo or end up with white blocks on either side. I usually leave enough space on an image for it to be printed in either size without using any important parts, but just in case, always doublecheck. If you have any questions about this, don't hesitate to contact me. I also have a little example here to help visualize what I'm trying to say here in rather technical terms. The transparent areas on the 5 by 7 and the 8 by 10 is what would be cropped. The 4 by 6 is the aspect ratio that I produce.