How will multisig addresses work? Will they consist of multiple traditional addresses, or will they have some different structure? Will one be able to specify whether they will require 2 out of 2, 2 out of 3 or M out of N signatures? Where can I find out more about their specification?
Transcation in bitcoin are actually scripts, where normaly it states one input and one output address. But there are other op code such as CHECKMULTISIGVERIFY script. Each signing party will have their own public key hence their own address. The way it works is as shown:
-N is the number of signature to continue
What the actual transaction does qould be set into the input script.
See here for an example of a transaction with a third party.