The Blockchain was the fundamental innovation that Bitcoin brought to the world. It is a database, or ledger, of transactions with linked validation data. Transactions and validation data get collected and compiled in blocks. Blocks get confirmed at regular intervals, after which you cannot change the history.
A Genesis Block is the name for the first block in a blockchain. It is unique in that its configuration is set by the developer. After this blocks are regularly added to the end of the ledger forming a blockchain. Once a block gets confirmed by the network you cannot change its contents. So a blockchain is; a chain of blocks containing transaction records that nobody can alter.
Where do the transactions come from?
In the case of Bitcoin the blockchain contains a history of bitcoin transactions. This is the movement of bitcoin units between accounts, or addresses, on the system. Anyone can create as many addresses as they want at will by using Bitcoin software. All addresses are empty at creation. Bitcoin units get introduced into the system through a process known as mining.
What is mining?
Mining is the primary method of blockchain security and generating bitcoin. Computers around the globe compete to confirm blocks of transactions. They do this by tracking transactions on the network and attempting to create a valid hash.
A hash is a cryptographic term for an ID that is output through a specific type of algorithm. This algorithm looks at a set of data and presents a unique ID consisting of numbers and letters. When hashing data any change to the data should result in a completely different hash. This way you can anyone in the world can reproduce the hash if, and only if, presenting identical data.