When Microsoft develops a new product, they assign a code name for the product. They begin to release dev versions prior to their final release. "Preview" is their early release. As bugs get reported they will fix them and continue to release updated versions. Until the final version is ready early this fall.
Don't install preview on top of Windows 8 on a computer that you intend to use for other than development purposes -- you can't restore it back to Windows 8 without losing all the data and registery settings. If you want to look at Windows 8.1 Preview then install it on another partition so that you can easily remove it later.
install it on another partition so that you can easily remove it later.
An alternate recommendation would be to install anything you are demo'ing on a dedicated guest VM. Take a look at VMWare and/or VirtualBox. Removing VMs is a lot easier than having to deal with partitions and boot managers, in my opinion.