I am a regular windows XP user but I’m very much fascinated with the features of Mac OS X. I saw the Mac Os first time when my uncle bought Mac book for him.
Now I’m wishing to use Mac over windows so I want to download it for free.
Please share some resources where I can find it for free…
You cannot get it for free. Nuff said.
Do we need to buy Mac? If so what is the cost for it (only os)?
You cannot buy only the OS.
Apple.com
And even if you could legally get a copy, you can’t just install OSX on any random PC. Apple is a media company with a hardware wing.
Actually, you can: Mac OS X 10.6 Snow Leopard - Apple Store (U.S.)
However, making the purchase would not help you, welcome_alexray. OSX is married to Apple hardware. There are projects out there which hack the OS to get it to run on non-Apple hardware, but they violate Apple’s EULA. Likewise, there are also projects which hack the OS to allow it to run on non-Apple hardware in virtual machines, but those also violate Apple’s EULA.
Hosting a VM of OSX on OSX is legal, but again, you would need Apple-branded hardware to support the host OS.
Bottom line, you will need a mac of some sort to run OS X. It doesn’t have to be new, though. There are used macs around available for purchase, but you should always be cautious about buying used electronics.
Except that is an upgrade. You need to have the hardware and existing MacOS installation available to use it.
But I guess its a matter of semantics really.
I guess I missed the fine print on that one.
I wanted to avoid, “Oh yes you can buy just the OS without the hardware, but it would be absolutely useless to you without Apple’s hardware…” I tried to avoid mentioning it for that reason. 
Might confuse the OP into thinking it might be like Windows where you can install it to anything. But I digress.
* And yes I am aware there can be circumvention of certain aspects of it that I will not discuss or mention here.
As others said, Mac only work with Apple computers. It is not that Apple computers have anything special but the do include a chip in the motherboard that is what makes possible to install the OS.
Hence, in an Apple computer you can install any OS but Mac OS only can be installed in an Apple computer