What is a Blockchain? A Quick Blockchain Definition  (credit : https://cryptobriefing.com/)

A blockchain is continually evolving list of records. For the purpose of cryptocurrency, it is a public digital ledger. The blockchain technology is important to Bitcoin and other cryptocurrencies because, without it, there’d be no verifiable way to prove that transactions were valid, or that funds were transferred.
The public and transparent nature of cryptocurrencies, in which these ledgers are shared globally and secured by incentivized parties, are some of their primary benefits. Applications range from personal finance and banking through to government records and company and customer records. Eventually AI and the blockchain will combine to create an instantly searchable and connected Internet of Everything, but that’s not here yet.
The notion of blockchains was conceived way back in 1991 when the cypherpunk movement was in full swing, but successful implementations of cryptographically secure digital currency (like Bitcoin or Ethereum) was still a long ways off.
In very basic, broad terms, all a blockchain means is that every change to a document (or in the case of cryptocurrency, a ledger) is tracked. The simple definition of blockchain technology doesn’t begin to describe its capabilities. This is more than a digital database. It’s a virtual supercomputer, a voting system, micropayments system, a bank, a peer-to-peer network like the world has never seen and so much more.
The Bitcoin blockchain, designed and implemented by Satoshi Nakamoto nearly 20 years later, was the first successful implementation of this distributed ledger idea.
The easiest way to think of a blockchain is this: each new “block” (of transactions or other data) must agree with the previous block. Lack of such agreement is evidence of fraud or other problems, and any decent cryptocurrency system will automatically reject such invalid transactions (or changes in cases where you are not talking about money).
That security is built in to the public blockchain technology and that rigid compliance has now spawned smart contracts that have almost limitless potential.Smart contracts can replace everything from company payments systems to selling a house. In each contract, the next step is only triggered when conditions are met, so the contract itself manages complex financial transactions. Private blockchains that can be verified by anybody on the system are finally viable with this new type of sourse code.
The clue to the nature of blockchains is in the name: every new block is essentially chained to the previous block, and every block that preceded it – and because the blocks are ‘distributed’, among several, (or millions) of computers and servers, no one entity can go back and change the history of those blocks (except in an extraordinary hypothetical circumstance involving taking over the majority of the network). 

The Bitcoin Model vs. The Ethereum Model

In the beginning there was Bitcoin, and its blockchain needed only to keep track of the movements of bitcoins. Along the way a few hundred alternative implementations of similar ideas were born, and many of these built their own blockchains. It is possible to find a record of every transaction made in Bitcoin or Ethereum (or the others, for that matter) — and while you may not at first understand what’s going on, you can certainly have a peak at either, here or here.
Then came the idea of “colored coins,” or blockchain tokens other than bitcoins, which have specific purposes. While Bitcoin is fully capable of handling these digital currency transactions, it can be prohibitively expensive. Thus was born Ethereum and a few others like it, all of which are intended to play the backbone/host to a multitude of tokens. Today, most new tokens are launched on one of these platforms.
Some implementations out of China, like Waltonchain and High Performance Blockchain, are intended for specific companies to run their own private blockchain with ease using their ledger technology.

We’ll Take The Blockchain, Keep the Bitcoin

Blockchain technology has far more uses than simply tracking units of account in a cryptocurrency. In fact, for several years while Bitcoin and others matured, blockchain tech was a preferred buzzword of the financial industry. While some people often summarily rejected the validity of digital currency, they could not deny the potential of largely immutable record-keeping (more here) and decentralization. The distributed ledger is here to stay.
The advent of Ethereum and its many tokens has led to the blockchainization of all things. Among the industries blockchain-oriented companies aim to disrupt are:

What Are The Benefits Of A Blockchain?

  • Security
Well-designed blockchains are more secure than private databases. Not only are there are multiple copies of the blockchain existing throughout the network in question, the more participants in a given network, the less possible it is to modify the contents of the ledger without being caught.
The scenario from the movie Office Space, wherein the characters surreptitiously shave off millions of fractions of a cent, is likely (and sadly!) impossible to pull off in a world where the finance industry uses blockchains. The distributed ledger simply will not allow it and smart contracts can take trust out of any business deal.
  • Transparency
While opacity has some place in financial institutions, transparency usually leads to less evil. Blockchains are by and large transparent by design, insomuch as the movement of funds and other data is concerned. For investors this can mean a vastly superior mechanism for overseeing the use of their money. For things like government records and other public information concerns, people gain an incomparable level of trust in the institutions providing data — be it something as simple as the latest revision of a regulation or something as important as a land title.
  • Resilience
Perhaps the biggest but oft-unmentioned benefit of the blockchain architecture is its resilience in terms of attack. The Bitcoin blockchain, for instance, has millions of copies around the world. As long as a few are left standing, these records are, for all intents and purposes, immutable.

Are There Problems Associated With Blockchains?

  • Large size
One drawback of the blockchain design is that it never gets smaller, only bigger, as more and more data is added. This is at the heart of a debate which eventually forked Bitcoin and is a key issue in any discussion of cryptocurrency. As of March, 2018, the Bitcoin blockchain stands at nearly 200 gigabytes. There is speculation that eventually it will become valuable to own a copy of the blockchain merely due to the transfer costs associated with downloading it.
Despite its size, the ledger of transactions can be accessed with “light” clients that do not require the whole of the blockchain to create and verify transactions.
  • Incompatibility
Blockchain-native applications and companies end up having to interface with legacy systems in awkward ways. The financial industry, much like governments, is slow to evolve. For this reason, adoption of blockchains will be a slow-going process for most firms, who’ll continue to use older technology in order to be most compatible with the financial tools they’re coming into contact with.

Why Are Blockchains Important?

“The key to overcoming the problems in […] central institutions is the block chain. Because, with the block chain, for the first time, we no longer need these central institutions for settlement, or for guaranteeing the value of coins, or for land titling. All of these functions can be replaced by a transparent public ledger that is safe from tampering, and which can make value and ownership clear and open for everyone… In turn, this spreads decision-making and the use of knowledge to a much larger number of people and institutions. The advantages of decentralization that are already being employed in private companies can then be felt society-wide.”
When you consider the global financial crisis or the occasional 10 extra years a man might spend behind bars due to bad record keeping, mismanagement, or even deliberate malfeasance, it’s not hard to see the basic applications where the blockchain can be helpful. Smart contract and ledger technology can simply our lives and make doing business easier and simpler. They open peer-to-peer systems that can cut middlemen out forever and provide personal finance for the world’s poorest people.
In short, blockchain technology can do everything. Now we just need to master this new ledger technology and make it work for us.