There were several attempts to create a digital currency before Bitcoin but the main reason why it has not succeeded is problem with duplicating digital money and spending same money more than once. Technology called Blockchain came as a solution to these problems and Bitcoin was the first project of creating digital currency that succeed and the main factor is blockchain technology.
The definition of Blockchain can be really tough. We can observe blockchain as a large data base. This technology is based on the consensus of people within the block and some transaction can go through the block only if all people in the block verify this transaction. Only when the block is completed, the next one is beginning. Blockchain technology is evolving every day more and more and as we already said in the text above it is main reason why Bitcoin which is based on this technology is the first digital currency that has succeeded.
As we said, transaction can be recorded in blockchain only if all people verify this transaction, so definition of blockchain is now a bit easier. Blockchains are lists of verified transaction and it keeps data safe. How? If you want to alter previous transaction you need to complete all transactions after the one that you want to change, and this is the way how spending same money more than once is prevented. Blockchain also guarantee security and safety as well.
Blockchain is great concept by itself, but can be also a platform for developing your own ideas and application, and the example of this is Ethereum. Network where you can trade goods and coins but also developing ideas, projects and other kind of investment.