Interestingly enough, you don't send Bitcoin to addresses. You send Bitcoin to a script of your design, and whoever can provide an input which causes that scrip

Interestingly enough, you don't send Bitcoin to addresses. You send Bitcoin to a script of your design, and whoever can provide an input which causes that script to evaluate true is allowed to spend those coins. This was originally meant to allow you to come up with all sorts of fantastic transaction types, such as coins which need M of N signatures in order to be released, or a Kickstarter clause that only releases the coins if a certain amount has been pledged. But, after the scripts had caused enough security vulnerabilities they were severely restricted. Clients will now only accept transactions which include one of the standard scripts.