Blockchain is essentially a system of recording and storing information such that it is very difficult or impossible to change or hack the information.
In other words the Blockchain is a digital ledger of transactions that once it is been recorded it is also duplicated and distributed across the whole network of systems on the Blockchain platform.
On the Blockchain, every block on the chain infrastructure contains a certain number of transactions.
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Whenever a transaction is carried out on the Blockchain, the details are automatically added to every participants ledger.
This is where the concept of decentralization comes in.
The database that is managed and run by multiple people around the world is referred to as the Distributed Ledger Technology (DLT).
Thus, the Blockchain is a kind of Distributed Ledger Technology in which the transactions are recorded with a hash ( which means an immutable cryptographic signature. )
The immutable cryptographic signature makes it difficult to tamper with the record, because if you make any changes on one chain, it will automatically become noticeable that it has been tampered with.
So, if a hacker wants to tamper with the system and get away with it, then they will have to change every block on the chain across the entire distributed ledger, it is impossible!
For this reason the Blockchain is considered a very safe and secure system.
Blockchain like that of Bitcoin and Ethereum are continuously growing because more blocks are added to the chain on a daily basis, and with more blocks also comes the complexity the system for any hacker to attack.
Blockchain is a breakthrough technology in an attempt to create a digital money.
Because for any digital money/currency to be worth anything or even accepted, it has to pass the trustworthiness test.
If people ( users ) do not trust the legitimacy of any digital currency, then it will be difficult for it to survive and be adopted.
This is why there is so much hype around Blockchain Technology, because it solves the problem of trust ( any one can verify that a transaction happened independently ) and also the transaction recorded are secure on the ledger and cannot be manipulated.
This is where Bitcoin as a digital currency comes in, because it was designed based on the principles of Blockchain Technology.
That is why Bitcoin can not be faked, hacked or double spent, this gives assurance to those that own this type of money that it has real value and it is secured.