The terms 32-bit and 64-bit refer to the way a computer’s processor (also called a CPU), handles information. The 64-bit version of Windows handles large amounts of random access memory (RAM)  more effectively than a 32-bit system.64-bit processors can represent larger chunks of data at a time than 32-bit  processors, allowing more efficient handling of all types of computer  calculations. Although 64-bit processing is inherently faster, it also requires  more RAM to accommodate the larger  chunks of data and provide better system performance.

32-bit operating systems can only use about 3.5 gigabytes (GB) of RAM, even if  you install more; 64-bit operating systems can use much more. For example, the  64-bit versions of Windows 7 Professional and Ultimate support up to 192 GB of  RAM. You need at least 4 GB of RAM in order to  experience an increase in system performance with 64-bit processing. If you have  less than 4 GB of RAM, moving to 64-bit could actually decrease system  performance.

Generally, 32-bit software is compatible with 64-bit versions of Windows, but  64-bit software is not compatible with 32-bit versions of Windows. Microsoft  notes that one exception to this is that many 32-bit anti-virus programs do not  work in 64-bit versions of Windows.

Also, it is not possible to upgrade directly from a 32-bit to 64-bit version of Windows or vice versa. If you want to make the change, you  will need to perform a clean install, which means that you will need to back up  your files and reinstall all your software.