What is the difference between a 32-bit and 64-bit processor?

Processor calls data from Memory i.e. RAM by giving its address to MAR (Memory Address Register). Selector electronics then finds that address in the memory bank and retrieves the data and puts it in MDR (Memory Data Register) This data is recorded in one of the Registers in the Processor for further processing. Thats why size of Data Bus determines the size of Registers in Processor. Now, if my processor has 32 bit register, it can call data of 4 bytes size only, at a time. And if the data size exceeds 32 bits, then it would required two cycles of fetching to have the data in it. This slows down the speed of 32 bit Machine compared to 64 bit, which would complete the operation in ONE fetch cycle only. So, obviosly for the smaller data, it makes no difference if my processors are clocked at the same speed. Again, with 64 bit processor and 64 bit OS, my instructions will be of 64 bit size always... which unnecessarily uses up more memory space.

First 32-bit and 64-bit are called architectures.

These architectures means that how much data a microprocessor will process within one instruction cycle i.e. fetch-decode-execute

In one second there might be thousands to billions of instruction cycles depending upon a processor design.

32-bit means that a microprocessor can execute 4 bytes of data in one instruction cycle while 64-bit means that a microprocessor can execute 8 bytes of data in one instruction cycle.

Since microprocessor needs to talk to other parts of computer to get and send data i.e. memory, data bus and video controller etc. so they must also support 64-bit data transfer theoretically. However, for practical reasons such as compatibility and cost, the other parts might still talk to microprocessor in 32 bits. This happened in original IBM PC where its microprocessor 8088 was capable of 16-bit execution while it talked to other parts of computer in 8 bits for the reason of cost and compatibility with existing parts.

Imagine that on a 32 bit computer you need to write 'a' as 'A' i.e. in CAPSLOCK, so the operation only requires 2 bytes while computer will read 4 bytes of data resulting in overhead. This overhead increases in 64 bit computer to 6 bytes. So, 64 bit computers not necessarily be fast all the times.

Remember 64 bit windows could be run on a microprocessor only if it supports 64-bit execution.

All calculations take place in the registers. When you're adding (or subtracting, or whatever) variables together in your code, they get loaded from memory into the registers (if they're not already there, but while you can declare an infinite number of variables, the number of registers is limited). So, having larger registers allows you to perform "larger" calculations in the same time. Not that this size-difference matters so much in practice when it comes to regular programs (since at least I rarely manipulate values larger than 2^32), but that is how it works.

Also, certain registers are used as pointers into your memory space and hence limits the maximum amount of memory that you can reference. A 32-bit processor can only reference 2^32 bytes (which is about 4 GB of data). A 64-bit processor can manage a whole lot more obviously.

There are other consequences as well, but these are the two that comes to mind.