I was going through mails and in one mail a person asked me, what is this 32 bit and 64 bit things and how do I know if my system is 32 bit or 64 bit. Which application my system would support and so on…
So, I thought better to document one small article which will answer those questions with minimal essential explanation of the terms involved using simpler way. I googled and collected few important paragraphs from various tech sites and posting it at one place.

i386 and i686 are both part of the x86 family of processors. They just refer to the specific age of the processor platform. i386 is an older platform (early 90s?) used back when 386 processors were used in machines. Then this was upgraded to 486 processors, which was the same basic instruction set as 386 just faster and newer. 586 was another upgraded and were when the term Pentium started floating around. Eventually all of these got encapsulated into the x86 architecture name. i686 just refers to the 6th generation of x86 architecture.

For all intents and purposes i386 and i686 is the same thing, just that i686 is a lot newer. They will have some additional instruction sets, but will be backwards compatible with i386. I would venture a guess that all of the processes you find today that use x86 would be classified as i686 processors.

x86 refers to a processor family. x86_64 is different than x86 (it is the 64 bit brother of x86). There are also Alpha processors, Sparc, ARM, PPC. These all refer to different processor families.

If you are installing an OS and a CPU is from the x86_64 processor family, then you cannot install an ARM based OS on that hardware.

Intel released the following processors on the dates shown

 
[vim]
$ /usr/bin/isainfo -kv
64-bit sparcv9 kernel modules

$ uname -m
sun4u

$ uname -m
x86_64
 
$ uname -m
i686
[/vim]
 
32-bit systems were once desired because they could address (point to) 4 Gigabytes (GB) of memory in one go. Some modern applications require more than 4 GB of memory to complete their tasks so 64-bit systems are now becoming more attractive because they can potentially address up to 4 billion times that many locations.
Since 1995, when Windows 95 was introduced with support for 32-bit applications, most of the software and operating system code has been 32-bit compatible.
Here is the problem, while most of the software available today is 32-bit, the processors we buy are almost all 64-bit.

So how long will the transition from 32-bit to 64-bit systems take?
The main issue is that your computer works from the hardware such as the processor (or CPU, as it is called), through the operating system (OS), to the highest level which is your applications. So the computer hardware is designed first, the matching operating systems are developed, and finally the applications appear.

We can look back at the transition from 16-bit to 32-bit Windows on 32-bit processors. It took 10 years (from 1985 to 1995) to get a 32-bit operating system and even now, more than 15 years later, there are many people still using 16-bit Windows applications on older versions of Windows.
The hardware and software vendors learnt from the previous transition, so the new operating systems have been released at the same time as the new processors. The problem this time is that there haven't been enough 64-bit applications. Ten years after the PC's first 64-bit processors, installs of 64-bit Windows are only now exceeding those of 32-bit Windows. Further evidence of this inertia is that you are probably reading this tutorial because you are looking to install your first 64-bit software.

Your computer system in three parts
Now we'll look at those three components of your system. In simple terms they are three layers with the processor or CPU as the central or lowest layer and the application as the outermost or highest layer as shown below:

To run a 64-bit operating system you need support from the lower level: the 64-bit CPU.
To run a 64-bit application you need support from all lower levels: the 64-bit OS and the 64-bit CPU.
This simplification will be enough for us to look what happens when we mix the 32-bit and 64-bit parts. But if you want to understand the issue more deeply then you will also need to consider the hardware that supports the CPU and the device drivers that allow the OS and the applications to interface with the system hardware.

What 32-bit and 64-bit combinations are compatible and will work together?
This is where we get to the practicalities and can start answering common questions.
The general rule is that 32-bit will run on a lower level 64-bit component but 64-bit does not run on a lower level 32-bit component:

These two tables illustrate the same rule:

 

The main reason that 32-bit will always run on 64-bit is that the 64-bit components have been designed to work that way. So the newer 64-bit systems are backward-compatible with the 32-bit systems (which is the main reason most of us haven't moved to 64-bit software).

An example of backward compatibility is Windows 64-bit. It has software called WOW64 that provides compatibility by emulating a 32-bit system. See the article How Windows 7 / Vista 64 Support 32-bit Applications if you want to know more. One important point that is made in that article is that it is not possible to install a 32-bit device driver on a 64-bit operating system. This is because device drivers run in parallel to the operating system. The emulation is done at the operating system level so it is available to the higher layer, the application, but it is not available to the device driver which runs on the same level.

Hardware virtualization is the exception to the rule
Another question many people have is whether a 32-bit system can run 64-bit software. As more people are looking to use 64-bit Windows they are wanting to try it out on their existing systems. So we are getting more questions about whether they can run it on their 32-bit processor or under their 32-bit OS.

Following the general rule, we would expect that you cannot run 64-bit software on a 32-bit system. Except that there is one exception called virtualization.

Virtualization creates a virtual system within the actual system. Virtualization can be achieved in hardware or software but it works best if the virtual machine is created in the system hardware. The guest operating system is not aware that there is a host operating system already running. This is the way that a 64-bit operating system can think that it is running on 64-bit hardware without being aware that there is a 32-bit operating system in the mix.

Tables 3 and 4 illustrate the result. Provided that the virtual machine can actually be created and isolated by the virtualizing software then the host OS is effectively removed from the equation, so I've grayed it out. We can now apply the general rules for a non-virtualized system to the three remaining layers.

 

Before you hurry away to try running 64-bit in a virtual machine, you must check that your computer BIOS supports hardware virtualization. If it does not then hardware virtualization will not work even if the CPU does support it.

Emulation of the 64-bit CPU is not an option
All the feasible configurations that we have looked at so far have the processors (CPUs) running software that use the instruction set that is native to that processor. Running 64-bit software on a 32-bit processor doesn't work because the 64-bit instructions are not native to a 32-bit processor.

But what if I could emulate a 64-bit processor using 32-bit software?

It is theoretically possible but practically impossible to emulate a 64-bit processor while running software on a 32-bit processor. Even if you can get non-native 64-bit emulation to work, the virtual machine that duplicates a 64-bit CPU would run very slowly because every 64-bit instruction has to be trapped and handled by the emulator. 64-bit memory pointers also have to be converted to work within the 32-bit address space.

Furthermore, my understanding is that the x86 (32-bit) processors used in PCs and Apple Macs are not able to completely emulate the x64 (64-bit) instruction set. Some 64-bit instructions cannot be trapped by the emulator. This causes the system to crash when the x86 processor tried to run those x64 instructions.