Someone Requests a Transaction via something called a wallet.
The transaction is send (broadcast) to all participation computers in the specific blockchain network.
Every computer in the network checks (validate) the transaction against some validation rules that are set by the creators of the specific blockchain network.
Validated transactions are stored into a block and are sealt with a lock (hash).
This block becomes part of the blockchain when other computers in the network validate if the lock on the block is correct.
Now the transaction is part of the blockchain and can not be altered in any way.