When you switch over to Ubuntu or even another version of Linux, you will encounter the home folder system. Different operating systems will store and configure files a bit differently so there are some changes to the way you use the folder system that you need to keep in mind.
If you come from the Windows mindset, which most users will, think of the home folder as your "My Documents." Your basic data will be stored into this folder for your user. If you have three different users on the machine, then there will be one home folder for each user. When you download files to the system, it is best to keep them all in this one location. You do have the option of putting your files into anything on the entire computer if you really wish to do so. By default there are folders there for pictures, music, and videos, but you can organize these however you wish.
Another part of the home folder is that it keeps your system settings in it for your applications. By default most of the time these settings are in hidden files. If you use your file browser and look at hidden files, Control + H by default, you will see a number of folders with a dot in front of the file name. They contain your settings for those various applications. Sometimes these settings can be edited from the folder itself. From time to time it is a good idea to backup these settings in case you want to install a different version of Linux or fear your hard drive dying and losing all your settings.
It is best to get into the habit of backing up your entire home folder since this is where all your files will be. Be sure you get all the folders and hidden folders. When you load a new version of Linux, it will be easy to configure your system with your old settings.