Is a bitcoin wallet address the same as your public key? The service CoSign shows that to create a multisig wallet you need to provide a Senders Address and Senders Public Key.   

I thought that they were the same thing.  

What am I missing here?
A public key in Bitcoin is a pair of 256-bit numbers used as part of the ECDSA scheme. A Bitcoin address is a Base58-encoding of the 160-bit hash of the public key (the hash function in question being RIPEMD160 of SHA256).

From the public key you can easily and deterministically compute the address, but from the address alone you cannot deduce the public key.