Every operating system is either 32-bit (x86) or 64-bit (x64). The differences between the two versions determine how well your operating system can use the Random Access Memory (RAM) of your computer. 64-bit versions make better use of your computer's RAM. 32-bit versions can only use a maximum of ~3.5 GB of your computer's RAM. Some programs are designed to only work with 64-bit operating systems, and not 32-bit. Here's how to tell which version of the operating system you have:
Right-click on the Start menu in desktop view and select System from the menu that comes up.
In the System Properties window that comes up, you will see your operating system's bit version listed next to System type.
Right click on the Start menu in desktop view and select System from the menu that comes up (Note: if you do not have a start menu icon, you may not have Windows 8 version 8.1. If that is the case, go to the tile view and start typing "system" to bring up a search for your computer. Select System from the results. Windows will perform this search of your computer even if you have not selected an area to start typing in).
In the system properties window that comes up, you will see your operating system's bit version listed next to System type.
Open the Start menu (the button with the Windows logo) in the bottom right corner of your screen, and find Computer in the list on the right of that menu.
Right click on Computer, and select Properties from the drop-down menu.
In the System properties window that opens, System type will tell you whether your computer's operating system is 32-bit or 64-bit.