Bitcoin: without capital letters it designates a virtual currency created in 2008. With a capital letter it designates the concept or the network itself. It is an autonomous monetary system that does not need trusted third party (bank, notary, etc.) to process transactions. These are validated by computers in a networked community around the world. No central bank is responsible for the creation of bitcoin, a currency that has no official price (there is no official quotation) or legal tender (no one is obliged to accept it as currency).
Blockchain: the "blockchain" is the technology behind Bitcoin. It must be seen as a large account book, a register, which lists in blocks all transactions since its creation. It is considered indestructible because it would have to do away with all the machines in the network. The blockchain is searchable by anyone on the Internet (but you will not understand much).
