HTML upload

This is the simplest method and the most tedious one at the same time. Image files are selected in the web browser - but only one at a time. Besides, you can often select only 10 files or so per upload process, which makes it even more time-consuming. The HTML upload is only recommended for orders of few photos. The advantage of this method is, that it's platform independent, ie works with any web browser and with any operating system. And it's easy to use as well.
Java upload / ActiveX upload
With the Java upload, files are uploaded with your web browser as well, but by using a so-called Java applet. This is a small software application which works embedded in a website and does not need an installation (However, Java has to be installed on your computer). This method is more comfortable than the HTML upload. Most of times, you can select many pictures and folders at once and then upload them together in one step. There are even some versions that compress the files so it takes less time to upload them. For Microsoft's Internet Explorer many printing services use ActiveX applets.
Software uploads
Often the most comfortable way to order photo prints on the internet is to use a software. Most of the printing services provide this software which has to be downloaded and installed. After that, photos can not only be selected conveniently (and you don't have to be connected to the internet while selecting which saves a bit of money for those who don't have an internet flatrate), but also edited. Some softwares are able to remove red eyes, and many can rotate and crop photos. Software uploads are recommended for larger orders and if you want to edit photos.