Firstly ,we see that What is blockchain?

A blockchain is a decetralised distributed technology.

A blockchain is a distributed ledger that is completely open to anyone.
The interesting property of blockchain is-: Once some data has recorded it becomes very difficult to change it.

During reading the definition of blockchain you have one question in your mind.
What is block?
Block is the important part of blockchain technology, Which carry information.

Let us see in details-:

[1]Data -:

The data stored inside a block depends on the type of blockchain.
For example,The ethereum blockchain stores the details about root node hashes of transaction trie,state trie and receipt trie.

[2]Hash-:

Block also contains ‘Hash '.You can compare a hash to a biometric.It identifies a block and all of its data and it is also unique,just like a biometric.

When block is created,Then at this time hash is being also calculated.When change in inside the data in block they causes the change in hash.

[3]Hash of previous block-:

In this technique the the block contains the address of previous block.Because of this the chain of block creates effectively and makes the continuity in chain.The first block of chain is unique they do not contain address of previous block,This block is called “Genesis Block”. Because of that this technique makes blockchain so secure.

Chevalier that will make the blockchain overprotective.

[1]Cryptography-:

It is the Cryptography who make blockchain transaction so secure.
Each transaction was signed unique private key.Every transaction can be further verified with a unique private key.
If the data changes between transaction ,the sign becomes invalid
As this condition block is avoided,And the cannot forward the chain.Then the transaction is abort,And secure from tamper.

[2]Decentralisation-:

Almost all of blockchain are distributed and decetralised.
Then the system is multi points,then it is harder to tamper.If hacker hacks One part of system this will not affect other part.
If hacker want's to hack the system, they can have require “55%-60% Legal control on system”. Only then somehow hacks the whole system.

[3] Unanimity (Consensus)-:

Every and each blockchain runs on Unanimous manner,Then it is prove that the every transaction happens with legitimate way.The common protocol of consensus is the proof-of-work,proof-of-stake And many more.
The general rule of ensure unanimity is long chain rule.It mandates that
Whenever two different versions of same block is surviving,Then the longest chain follow to whom is considered as authentic.And other is bogus.

Weaknesses

“Wherever in the world not any system is perfect,They have cons and pros.”

[1] Cheating with system-:

The tamper can deceptions the nodes in trash time by having miners already solved cryptocurrency puzzles in the “selfish miner” technique.
The other popular attacks are-:

1]Sybil Attack.
2] Routing Attack.
3]Denial of service attack.

[2] External Contact-:

The many more major issues raised by vulnerabilities.In many cases this found that the it is created by user.For example The frail interaction link cause the $460 million(2014 value) bitcoin lost in the heist.

[3]Other weakness-:

We use third party wallets to store cryptocurrency.But this wallets are safe?.Because of in many cases is seen that the third party digital wallet and keys storage system steal the personal data and use it for their benefits.

Conclusion-:

We don’t say that the blockchain technology is bad or worst.In the our world ‘Nothing is perfect'.Everything is cons and pros.
But our conclusion is say that the blockchain technology is better technology as compared to traditional.