Too vague of a question.

Ans!

This is the best answer I can provide with such a open question. Try this link from Microsoft and see if it gives you some info about the difference. The biggest difference is that 64 bit OS and applications are able to use greater amount of resources. For example you put 8GB of RAM in a laptop with Win7 32 bit, it can only use 4GB roughly as max however a 64bit OS can allocate all of that memory for things like applications and services and whatnot.

http://windows.microsoft.com/en-US/windows-vista/32-bit-and-64-bit-Windows-frequently-asked-questions

32bits

This post should help you:

http://msiworld.blogspot.com.au/2011/09/creating-msi-for-x64-and-x86-machines.html