From IThelp

What does it mean that something is 64-bit and why had the IT-department decided to go over to this on Windows 7?

A simple explanation is that is has to do with the fact that data processors only works with binary numbers (abbreviated bit), meaning combinations of ones and zeroes. A bit can have to conditions (1 or 0), two bits can have four (00, 01, 10, 11), three bits can have eight (000, 001, 010, 011, 100, 110, 101, 111) and so on. Eight bits (or octet) is traditionally called a byte, giving 28 combinations of zeros and ones. A 32-bit processor works with 232 combinations, and a 64-bit processor works with 264. That means that the 64-bit processor works faster and more efficient with heavier applications. Another important characteristic with 64-bit processors is that they can utilize more than 4GB of memory, which is the maximum limit for 32-bit processors. Today 4 GB of memory is more than enough for regular use, but in the long run it will probably require more in different context.

All newer PCs here at the University have 64-bit processors. These are compatible with 32b-bits systems such as Windows XP, but will not be able to take full advantage of it that way. Previously it's been a lack of machine drivers for 64-bits systems, and some programs have had problems functioning as they should. Therefore (for more practical reasons) it has been most appropriate to run 32-bit systems. Today however, 64-bits systems have become so common that most of the new hardware and software are being developed with this support. Therefore, it's natural for us to transition to 64-bit edition of Windows 7.