Disclaimer: Bitcoin noob here
Pools are collections of people who mine Bitcoin. When a block is discovered by someone in the pool, the reward is shared amongst everyone who is in the pool relative to their contributed hashing power (often minus a small fee for the pool operator).
Ghash.io is the largest pool, and the member of the pool control a good 35-45% of total hashing power on the Bitcoin network. A pool with >50% the total hashing power is bad, because the operators in charge of the pool could carry out malicious acts on the Bitcoin network without any members of the pool being aware they are contributing to these acts.
If you control a significant percentage which is less than 50%, you can attempt to do this but it is both uncertain to work (on any given attempt) and costly if it fails. Note that you can take many bites at the apple if you're willing to suffer the opportunity cost, so if you have a sufficiently profitable transaction to cheat on, you'll come out ahead.
The most vulnerable players in the Bitcoin system currently are gambling sites, but in principle with sufficient hash power you can do this to anybody using Bitcoin. Gambling sites are particularly at risk because their intended behavior is swapping bitcoins for bitcoins very quickly, and if you can doublespend, you can swap bitcoins for bitcoins but then say "You know those bitcoins I sent you? Psych, I didn't have them, even though you thought I did." after you've been told that you lost a bet. If you win the bet, you simply don't rewrite history to invalidate your bet. Repeat as desired.
This is the planned and anticipated vulnerability in Bitcoin. Great news: it's rarely the planned and anticipated vulnerability that kills a system.
+ Double spend attacks are not the only attacks you can envision. For example, if you control 51% of hashing power for a month, you're capable of essentially invalidating all transactions globally in the past month, at a time of your choosing. Like, if an unrelated party Bob had been paid in bitcoin by his employer two weeks earlier, and Bob then attempted to transfer bitcoins to Mt. Gox to change into dollars, Bob's bitcoin client would suddenly tell him "Dude, you might remember that salary payment, but it never actually happened." and Mt. Gox would say "Umm, you don't have bitcoin to transfer in, what are you talking about?"
What would happen, if due to a war many of the datacenters on earth were destroyed and we would only have 1/4 of the computing capacity left. Would sending BTC become impossible, or is mining not possible then?
In theory everyone mines alone to prevent this, in practice, this isn't very feasible.
+ This phrasing is rather more robust than I'd prefer, but you're dangerously mistaken about something which many people will attempt to get you to equate with money.
The Bitcoin protocol does not trust owners of large amounts of bitcoins. An emergent behavior of the protocol is that it trusts people with large amounts of hash power over given periods of time. Why not trust every client equally? Because Satoshi believed that IP addresses were really easy to conjure up, and compute farms were really hard to conjure up, so a malicious actor could easily get a majority of the network if you were counting via IP addresses but it would be hard if you required computational proof of work.
This is one of the core engineering decisions in Bitcoin. One of the other ones is, to incentivize people to spend computational power on computations which have no utility the overwhelming majority of the time, periodically they're allowed by the protocol to claim bitcoins which are created from nothing. Every bitcoin in circulation first came into possession by someone who won a lottery with tickets basically bought by the expenditure of hash power.
Response to edit: In a war during which 75% of the world's datacenters are destroyed, the Internet collapses, Bitcoin dies along with many hundreds of millions of people, and cryptocurrency enthusiasts join Beanie Baby collectors in the long line of people who will not find their favorite hobbies treated well in a nuclear winter.
If you're answering purely as a math abstraction, though, the protocol will eventually self-correct by picking a new, lower block difficulty, with the goal being creating a new block approximately every ten minutes. The approximate upper bound on how long it would take to recalibrate if you lost 3/4th of the hashing power is plus or minus eight weeks. (I originally said "an hour", which is clearly in error.) During the interim, Bitcoin would be possible to transact in and mine but it would be slower than previously -- instead of it requiring about an hour to know that a given transaction was safe to rely on, it would require about four hours.
But again: Bitcoin is very much not robust against "the end of the world."
Can anyone "lucky" enough receive 25 BTC from nowhere, is that what you mean by `25 BTC reward`?
How can people share a "computer cluster"/minin pool (that is what you mean right?). Do you mean that one person or company owns such a huge cluster and let's all members share 'virtually' a single 'Bitcoin wallet'?
I'm not really that familiar with it, sorry for the nooby questions.
A lone miner has a very low chance of finding the value, but will get the full 25 BTC. In a pool, a number of miners combine their computational power, and if the pool finds the value they get awarded a fraction of the bounty corresponding to the amount of work they did (the number of possible values they checked). E.g. if A, B and C are in a pool, A verified 140 values, B verified 100, C verified 10 and one of them (who it was isn't supposed to matter) they'll get respectively 14, 10 and 1 BTC. If they had not been in a pool, one of them would have gotten 25 and the others 0. So being in a pool reduces your payout, but increases the chances that you get a regular payout.
I'm handwaving here, and this is how I understand it.
Yes. It is verifying transactions. Making sure that transactions are valid and people aren't double spending or spending money they don't have. As a reward for doing this miners who find blocks generate bitcoins from nowhere, the reward is currently 25 bitcoins for finding a block. It is luck based, you have to find the right hash. You just keep hashing away until you find the right one, I believe.
It's called "mining" because it's a reference to say mining for gold or other items of value. You can look in the right spot, but there's a bit of luck involved.
>How can people share a "computer cluster"/minin pool (that is what you mean right?). Do you mean that one person or company owns such a huge cluster and let's all members share 'virtually' a single 'Bitcoin wallet'?
More or less. They rewards are divided based on how much computing power you put toward finding that block (shares). Once it is found the rewards is divided up proportionally. In reality some pools have slightly different reward schemes, but I'll make it simple here. So, if the reward is 25 bitcoins and I put in 10% of the "effort" to get that 25 bitcoins, I would get 2.5 bitcoins even though I am not the one who found that block, someone else in the pool did. That lives on the pool's wallet. I then go and log into the pool's website and transfer those bitcoins from the pool to my wallet. Some pools have an auto transfer so that when you get to a certain amount it automatically goes to your own wallet. Those coins do me no good at all in the pool's wallet, until they are transferred to my own wallet.
Pools aren't run by companies, not that I've seen. They are mostly run by anonymous operators.
It is long, but I strongly recommend reading
http://www.michaelnielsen.org/ddi/how-the-bitcoin-protocol-a...
See here for a more technical explanation:
Nobody mines on their own, unless they are running a server farm of mining gear. It would take too long to make a profit, and you might not ever find a block by yourself, depending on your hashing power and luck. Finding blocks are what generate coins. People combine their hashing power and mine in pools, that's what Ghash.io and BTCGuild are. These pools take all the profits and divide it up to all its members based on how many shares they submitted to find that block (basically, their mining power). Pools allow a more or less consistent payout for their miners instead of mining forever and getting nothing then once in a while getting a big windfall. These pools are run by an operator usually anonymous. These pools sometimes (usually) take a small fee, and some people donate as well to keep the pool running.
Pools are subject to DDoS and other attacks as well often.