I think i am beginning to understand what you mean.... it's like if i have 2 external HDD so if one of them goes corrupt or something else then i stand a risk of losing all the data in that disk... but if i have a NAS unit ieven if the disk goes bad i just have to replace it and all my data will be still there... right?
If you configure things correctly, yes, that is right. Different RAID configurations can handle different levels of loss, while also improving the performance of your disks by using them all at the same time. For example, the most common RAID level is RAID 5, where you can lose a single disk and still keep all your data. In RAID 5, one disk is kept on stand-by as a spare. You write to every disk simultaneously (though you don't get any extra performance from the extra disk), but you don't read from the spare disk unless one of the disks dies. So, if you have four disks, your performance can reach up to three times the read/write speeds of a single disk.
Basically, RAID gives you the advantage of failure tolerance and performance enhancement for the tradeoff of losing a certain number of disks worth of disk space. You can read more about it on Wikipedia.
And what is your NAS configuration?.... and how much did it cost you?.. if you could tell me how to assemble it i am more than willing to look into the prospect of buying an NAS unit myself. After all my data would be safe even if my disk goes bad. 
I built a small computer for my first NAS. It consisted of an Intel Atom board and CPU (about $90), 1GB RAM (about $30), one 500GB boot disk (about $50), four Samsung 1.5TB disks (about $180 each), a cheap 4-port RAID card (about $20), and a cheap case (about $50). Prices are what they were at the time (about two years ago) and in CAD; obviously they have dropped considerably since then. I used software RAID through Linux so I did not use any sort of hardware-based RAID. It acts essentially as an expansion for SATA ports, since the board only had two SATA ports. I went with RAID 5, which gave me 4.5TB (about 4.1TiB) of space.
These days you can get Atom-based mobos for roughly the same price but with considerably better performance, not to mention everything else is a lot cheaper. You can get 3TB disks for less than $180.
My current server cost considerably more, because I was looking to build a server that handles more than just NAS functionality. My NAS is actually still running, sitting on the network, seeding a small number of files to oblivion, but fileserver, web server, and general usage tasks have been taken over. When I first got it, I got an Intel i5 2400 ($200), Gigabyte mobo ($130), 8GB RAM ($75), 60GB SSD ($120), pedestal server case ($60), and 6 Hitachi 3TB disks ($120 each, minus $10 rebate). I got lucky with the disks and managed to buy them a couple months before the big Thailand flood that caused HDD prices to skyrocket.
(Note: All prices are approximate)
Since my initial purchase, I have replaced the SSD because it failed on me. The new one is 120GB and SATA III, much faster and more reliable than the old one; cost me around $200. I've upgraded the RAM to 32GB, which was also around $200.
Anyway, it definitely goes without saying that prebuilts are quite expensive and commissioning someone to build something for you is also expensive.
You buy say a 3tb drive pop it in and you have 3 tb of storage. when you start running low you drop in another 3tb drive you now have 6tb of storage. anytime you need space you just add a drive to it it will do all the work of expanding the file system and such, if you run there hybrid raid 5 then you need to start with at least 2 disks
That's pretty cool. It builds and rebuilds the array as you expand? Does it do something similar to Drobo?
