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Although multi-signature has become possible in early 2012 because of BIP11, the large-scale acceptance of multi-signature is thanks to another type of transaction: payment to script hash, or P2SH for short. With this new transaction type, any script can be used to verify the transaction. Prior to this, only specific scripts could be strictly used to verify transactions.
Since any script can be used, the maximum number of keys that can be used in a multi-signature script has been increased from 3 accepted by BIP11 type multi-signature to 15 compressed keys and corresponding signatures allowed by P2SH scripts.
Most importantly, P2SH has added a new Bitcoin address model. Under BIP11, simply provide someone with a multi-signature address and conduct a transaction with him. You must tell the other party exactly how to set up your multi-signature to punch coins: what are the secret keys, how much is needed to verify the transaction, and how Initiate multi-signature...
P2SH reduces the difficulty of using multiple signatures to a level equivalent to using a single public key. Using this new technology, a large number of online wallets and software have risen, while improving the security of the Bitcoin network.