Simply put: regular bitcoins only need to be signed by one address (private key) in order to be spent. If coins are encumbered in a multi-signature transaction, it requires multiple signatures -- perhaps multiple, different, geographically separated computers. Or multiple people. Perhaps 2 out of 3 owners of a company will need to supply signatures to send the coins.
There's a very rich set of functionality that can be enabled through multi-sig. Escrows, contracts, I can't even fathom all of them myself. But the key is that there is no longer a single point of vulnerability for multi-signature-required coins. An attacker will have to compromise multiple computers/people/nodes/servers in order to steal those coins.
EDIT: there's other features of multi-sig that might actually make it easier to spend [allow any one of multiple people to access them], or produce escrow such that defending against an attacker is not exactly the intent. But I expect that the most common use-case will be for regular users to split their private keys between two devices (such as primary computer and smartphone), such that both devices need to be compromised for the attacker to get the coins (and the user will have to access both devices to use it).
Unfortunately, all this comes with a lot of extra complexity. But it's up to application developers (like me), to try to make it useful for non-Bitcoin-experts. And I look forward to digging into it after Armory becomes beta.
So, soft is not ready, I know it, but technically it is possible now right?
Is it possible to make transactions like this time or block dependent?
Example transaction:
Person A borrows from me 1000 BTC but as insurance he have persons C, D, E who pays 20 BTC each to ensure me that person A is worth my trust.
Person A have to send me back 1010BTC (with interest) before next Friday (block number), and if he will not, automatically C, D, E will lost their 60BTC and I will get that.
If A will send me BTC back in time C, D, E will get their money back.
Is it technically possible now?
If time relation is not possible, we could engage some third party as time responsible and trustworhy company with some satoshi-s profit.
