user323: The Bitcoin API call of createmultisig returns two parameters - address and redeemScript. What is the latter used for?

user4139: To really understand this, you would need to read the protocol specs.  But I'll describe it briefly.
Each transaction contains a script.  In order to claim the bitcoins from a transaction (e.g.: you want to spend them in a new transaction) you have to provide whatever proof is described by the script.  
Normally, the script is pretty much the same: the intended recipient has to provide a digital signature that proves they own the destination address.  But you can requires a lot of different kinds of proof: require more signatories, no proof at all, or require the claimant to solve a puzzle.

user11401: It has to do with this: https://en.bitcoin.it/wiki/BIP_0016.
Instead of providing the necessary scriptSignature when sending coins, you can just provide the hash of the script, and let the redeemer provide the actual script upon spending.

323: <p>The Bitcoin API call of <code>createmultisig</code> returns two parameters - <code>address</code> and <code>redeemScript</code>. What is the latter used for?</p>
 4139: <p>To really understand this, you would need to <a href="https://en.bitcoin.it/wiki/Script" rel="nofollow">read the protocol specs</a>.  But I'll describe it briefly.</p>

<p>Each transaction contains a script.  In order to claim the bitcoins from a transaction (e.g.: you want to spend them in a new transaction) you have to provide whatever proof is described by the script.  </p>

<p>Normally, the script is pretty much the same: the intended recipient has to provide a digital signature that proves they own the destination address.  But you can requires a lot of different kinds of proof: require more signatories, no proof at all, or require the claimant to solve a puzzle.</p>
 11401: <p>It has to do with this: <a href="https://en.bitcoin.it/wiki/BIP_0016" rel="nofollow">https://en.bitcoin.it/wiki/BIP_0016</a>.
Instead of providing the necessary scriptSignature when sending coins, you can just provide the hash of the script, and let the redeemer provide the actual script upon spending.</p>
