Chain of Blocks (Blockchain Linking)
Blocks are linked by each one containing the hash of its predecessor, forming an unbroken chain back to the genesis block.
In depth
This linking is why the structure is called a chain. To alter block 100 you would have to re-mine block 100 and then every block after it faster than the honest network adds new ones — an astronomically expensive task. The longer a block has been in the chain, the more work sits on top of it.
Why it matters
It explains why confirmations increase security: each new block "buries" your transaction deeper.