The script contains two components, a signature and a public key. The public key belongs to the redeemer of the output transaction and proves the creator is allowed to redeem the outputs value.

So it's the receiver's public key. How does that help? – Phlexonance – 2013-03-19T12:09:52.473

3No, the script contains the spender's public key. It is like saying - "here is a check to address X. I own that address, here is a proof: I have a public key Y, it turns into address X, and here is a signature from my private key that can be verified with Y". The sender proves that they know the public key and that they posses the private key. An address is just a way of saving space. – ThePiachu – 2013-03-19T21:50:20.090

1the input transactions i have looked at just contain the signature and no public key. i'm guessing that the public key comes from the output of the previous referenced transaction being spent (located using the previous output hash and index number). is this correct? – mulllhausen – 2013-12-21T05:41:02.250

@mulllhausen How could that possibly work? If the previous transaction were from A to B and C, it wouldn't contain the public keys for B and C, just the addresses. So when B wants to spend that input, they must put their public key in the transaction. – David Schwartz – 2014-01-17T04:32:21.720

@DavidSchwartz it seems to be the case when spending a coinbase transaction as i understand it. take a look at this question for an example - http://bitcoin.stackexchange.com/questions/19081/parsing-bitcoin-input-and-output-addresses-from-scripts - but please correct me if i am wrong.