May I understand, why there is a panic of 51% hash rate reached, I still can't get the point.
Suppose u & me has made a deal that I'll give u 1 BTC and u'll sell me a website. Now, if I have understanding with the 51% hash power holder GHash.IO, I can send u 1 BTC, take your website's credentials, again send 1 BTC to GHash.IO. GHash.IO will accept the 1 BTC I sent to them and not u. Now they can send me back 0.5 BTC. So, in effect I get my job done with 0.5 BTC, whereas I am supposed to pay 1 BTC.
I didn't get that to the end. I just want to understand the 0.5 and 1 BTC.
Again please, if you don't mind.
In simple term, the 51% network hashrate provides monopoly (51% probability) to the miner to mine blocks. So they decide which Tx to include in their blocks and which not. Now, they can write code in their system to completely filter out your Bitcoin address so that any incoming Tx to your address will never get confirmed. So the person sending U bitcoin can re-use their coins. This is called double spending, where your coins never get confirmed because of the bad miner with 51% power.
While this paragraph vaguely hints at what a 51% attacker could do, it's far from correct.
It's true that someone can update their software to not include certain transactions. However, unless that person control 100% of the network hashrate, eventually another miner will find a block which does include a transaction that is being ignored by the attacker. This may cause some confirmation delays for the target, but that's it.
Now, a miner/pool with >50% of the hashrate can go further than that and ignore all blocks mined by others and only work on a blockchain that consists entirely of blocks mined by the pool itself. Only such an action would allow the attacker to completely prevent certain transactions from confirming. But such behaviour would be noticed by others almost immediately as other miners start seeing all of their blocks orphaned.
Another possible attack is for the attacker to spend some of its coins in exchange for something else (fiat, altcoins, cookies, whatever), wait for the transaction to be confirmed and then attempt to replace the blockchain from the point of the first confirmation of this transaction up until the current point with a new version that instead spends the transaction to another address. This can be done either by the attacker starting to mine a secret fork that it doesn't release to the public until the transaction has received enough confirmations and the purchase is complete or by the attacker only starting to work on his alternative blockchain after the purchase is complete.
The first option means that all of the attackers hashpower suddenly becomes "dark" and the speed with which blocks are mined is cut in half. Finally, when the secret blockchain is broadcast, this will orphan a chain of 3-6 (or even more) blocks, which will immediately raise eyebrows. The second option takes a long time to do, since catching up with a network that is 6 blocks ahead and is hashing with almost as much power as you can potentially take quite a while. When the new chain is broadcast, this'll cause an even longer orphan-chain than in scenario 1 and this'll definitely be noticed directly.
A 51% attack is not a subtle instrument. If used, it will get noticed almost immediately and consequently there will probably be a strong drop in the Bitcoin price.