64-bit versions of Windows can run 32-bit software, but 32-bit versions of Windows canâ€™t run 64-bit software. 32-bit programs can only use 2 GB of memory each, while 64-bit programs can use much more. A 32-bit processor is also 32-bit based, in that they work with data units that are 32 bits wide. A computer with a 64-bit processor can have a 64-bit or 32-bit version of an operating system installed. However, with a 32-bit operating system, the 64-bit processor would not run at its full capability. 64-bit processors process more data more quickly.
32-bit software will be more compatible with more machines but will generally not run as fast as 64bit due to the processing bit limitation and also the capped memory. If your application uses a lot of memory (lots of images, sounds, etc.) you should try to plan on having a 64 bit build available.
More good info:
http://www.howtogeek.com/194119/why-are ... f-windows/