Miners don't contribute to the validity of the currency - they verify that transactions are valid and recorded for posterity. Say you want to spend a coin by sending it to someone else. You do this by announcing your intention to the network. The network then has to verify if that coin is actually in your wallet, that you have authorized the transaction and that you haven't spent the coin previously. Without such checks people would be able to spend coins they don't own or spend them multiple times, which is obviously unacceptable.

The role of verifying and recording all transactions is fulfilled by the miners. Anyone can use Bitcoin, even without being a miner, but the miners make sure that there is no funny business. Since all transactions are public it is known at all times how many coins there are and who owns them. Miners use this information to make sure everything is in order. A transaction remains pending until it is verified by the network of miners.

So why would anyone volunteer their computing power to be a miner? Because new Bitcoins are created from scratch every 10 minutes by the network and given to one miner as a reward for helping maintain the integrity of transactions. So who gets these coins? The first person to solve a difficult mathematical problem and announce the solution publicly - this is the "proof of work". It's like playing the lottery, with a prize being given out every time a new block of transactions is recorded. The more computing power you dedicate to mining, the higher your chances are of being the first person to solve the problem and "winning" those Bitcoins.

That's a very comprehensive answer, and thankyou for it. However it does still leave a few questions:

1) Without miners, would the currency actually be viable?

2) Given BitCoins will one day run out (if not mathematically, surely on return on investment), surely the miners will one day stop? What then?

3) what is the "lead time" on transactions given they are community validated. I always assumed a BitCoin was immediately transfered, you're suggesting (I think) it's not valid until then ratified by the community.

Bitcoin is designed to be a fixed amount of coinage in existance at a predetermined point in the future. Supposedly, if the monetary base gets fixed, it fights off inflation pressures affecting the currency.

I disagree with this principle since it does not consider deflationary pressures.

I don't trust the currency simply because it is a digital currency and give the high level of skill that hackers nowadays have, I have no assurance they can't steal the bitcoins easily, or replicate the coins or do whatever those high skilled programmers can do, to take advantage of a digital coin where there is no central authority governing it.

1) No, without miners there would be no bookkeeping anymore, so Bitcoin would cease to exist.
2) Bitcoins will not run out, but at a certain point no new coins will be minted anymore. This is scheduled to happen in 2140. At that point, 21 million Bitcoins will be in existence, assuming that Bitcoin is still around at that point. The system would eventually have to switch from new Bitcoins to transaction fees as a reward for miners in order to maintain an incentive.
3) Validation of transactions takes at least until the next block is verified, which happens every 10 minutes. Verification can take longer, though.

"Assume", "might", "would"... What are you talking about my friend? What you described, is exactly how it DOES work, and it has been working like that for years now. There are tens of companies, that do exactly what you described. Coinbase and BitPay are the most popular ones.

That is exactly how wordpress.com and all others are able to set prices in USD, accept payment in bitcoin, and receive the exact amount in USD into their bank account each day. Doesn't matter the price of bitcoin, they set their prices in USD, and they get the exact amount in USD (or EUR or any other currency), while their customers can use bitcoin to pay.

I saw Bitcoin on gbatemp website said Bitcoin is official dead now. Bitcoin should remove from all other websites.

Not dead, but heavily damaged. One exchange has failed, but it was the largest one, and some $300 million in book-entry money has vanished. As more credible reporters have put it, it is about as damaging to Bitcoin as the New York Stock Exchange going insolvent would be to the securities market.

The value of a currency is in the credibility of its backers, not in any real or artificial scarcity or in any algorithm or process, and Bitcoin has lost much of that credibility.

It remains to be seen whether a stampede will finish it off entirely. Traditional investors have a way of stampeding when even a partial failure of a financial system occurs. Bitcoin speculators may be sufficiently ignorant of history that they will not repeat this mistake. But I doubt it.

It will be interesting to see whether their blind faith postpones a massive bitcoin crash.

I say postpone, because eventually, there will be a massive crash. It's inevitable.

Bitcoin is not a currency.
The US Dollar is a currency.
Bitcoin is a commodity, and a virtual commodity at that.

Plussed for talking sense.

While it's a pity an exchange went down I thought the whole point of Bitcoin was to promote personal control, that people should keep their digital wallets on them and not on some site where you risk losing access. I know it's convenient and all but that might be the tradeoff.

"Personal control" of your money (or something that makes a pretense of being money) is an illusion. Without institutions that will validate it and exchange it, merchants that will accept it in trade, it is so much metal, paper, book entries, or bits, without any kind of value other than scrap.

I suppose but a government can't freeze your bitcoin account? It's about direct access then a secondary access through a service that you need to trust.

Interesting thought - I'd have thought that in the UK and US at least that Bitcoins would be freezable assets under anti-money laundering and anti-terrorism legislation.

In practice, this would probably be achieved by confiscation of electronic equipment.

A government could seize computers used to access the account. It is likely this will happen to at least some exchanges in the near future.

If I have coin or paper money in my pocket, and I want to pay for goods with it, any merchant sufficiently skilled in his trade can determine without the need to interact with any other person or thing that my money is good.

Bitcoin transactions require confirmation by a network of computers. The need for the availability of the network creates a point of failure that can render bitcoins useless.

And money that you cannot prove is yours, and that a merchant cannot prove is genuine, is scrap.