Have you ever wondered why you can't address more then 4GB of memory in Windows, or some other 32-bit version of an operating system (e.g. OS X or Linux). This is because most modern computers have reached or exceeded the limit of the maximum addressable memory of a 32-bit operating systems which is 4GB. To address this problem operating system manufactures have released 64-bit versions of their OSs (e.g. Windows, OS X, and Linux).
To run these new OSs, your hardware has to be 64-bit ready, most modern computers made in the last few years can support 64-bit OS (with the exception of Netbooks). Most older 32-bit applications will run on these newer OSs without problem, but to be able to fully utilize the capabilities of the new operating system you must update your applications to the 64-bit versions.
Differences between 64-Bit and 32-Bit
For a brief technical explanation of the differences between 64-bit and 32-bit computer architectures:
As operating systems have been upgraded to deal with larger resources, so have other technologies evolved to handle new advancements: