A bitcoin address is a derivative of a hash of a private key.

- http://en.bitcoin.it/wiki/Technical_background_of_Bitcoin_addresses

 - http://en.bitcoin.it/wiki/Private_key
 - http://en.bitcoin.it/wiki/Address

A transaction requires cryptographic signature(s) made using the private key(s) of the input addresses:

 - http://en.bitcoin.it/wiki/Transaction#Principle_example_of_a_Bitcoin_transaction_with_1_input_and_1_output_only

A block includes a reference to a prior block and includes zero or more transactions that have occurred since that prior block:
 - http://en.bitcoin.it/wiki/Blocks