A blockchain, originally block chain, is a growing list of records, called blocks, that are linked using cryptography. Each block contains a cryptographic hash of the previous block, a timestamp, and transaction data (generally represented as a Merkle tree).
By design, a blockchain is resistant to modification of the data. It is "an open, distributed ledger that can record transactions between two parties efficiently and in a verifiable and permanent way". For use as a distributed ledger, a blockchain is typically managed by a peer-to-peer network collectively adhering to a protocol for inter-node communication and validating new blocks. Once recorded, the data in any given block cannot be altered retroactively without alteration of all subsequent blocks, which requires consensus of the network majority. Although blockchain records are not unalterable, blockchains may be considered secure by design and exemplify a distributed computing system with high Byzantine fault tolerance. Decentralized consensus has therefore been claimed with a blockchain.
The main goal of PoW is to deter denial-of-service attacks (DDOS) by requiring (computational) work to be done before block submission. This means that, in order to mine a Bitcoin block, you have to spend/have computational power/energy.
Top PoW Coins: https://coinmarketcap.com/view/pow/
Proof of Stake (PoS) is another consensus structure, like PoW, but instead of requiring work or computational power, it requires the miner to hold a large amount of the cryptocurrency. Let's say cryptocurrency "Stakeproofcoin (SPC)" relies on Proof of Stake, and that Bob owns 2% of all of the SPC in existence. This means that the probability of Bob mining a proof-of-stake block is 2%. Proof of Stake aims to prevent attacks by having high requirements for such an attack (like PoW) and by lowering incentives dramatically. In order for Bob to attack SPC, he would need to own a majority (51%, let's say) of all SPC coins, which would require a lot of money in the first place. Also, his incentive to attack is very low, since if he does attack SPC he would suffer dramatically from his own attack (because he owns so many SPC).
Some people, like Ethereum supporters, think Proof of Stake is more efficient and fairer compared to Proof of Work for reasons such as the above. Additionally, anyone could become a PoS miner technically given they had some amount of the coin, however getting into Bitcoin mining generally requires expensive hardware and commitment.
Top PoS Coins: https://coinmarketcap.com/view/pos/
Delegated Proof of Stake (DPoS) is a newer consensus structure, and is actually behind many cryptocurrencies including Steem. It's somewhat similar to PoS but has different and more "democratic" features that some say make it more efficient and fairer. Users of a DPoS crypto vote for delegates/witnesses to serve on a panel of witnesses. Lots of variables within the network such as transaction sizes and fees are controlled by this witness panel. The witness panel also (deterministically) selects witnesses to "witness" or confirm blocks. Clearly, the network and users place a lot of trust in these elected witnesses to give them such powers, yet if they misbehave, they can easily be voted out of the "witness panel," so there is really no incentive for them to misbehave or upset their voters.
To add in the "proof of stake" part of DPoS, stakeholders have influence proportional to their stake in the system. DPoS is found to be more decentralized than other consensus systems because the threshold to enter is very low. Other alternatives generally "allow" anyone to enter, but most people are excluded from entering due to high costs/needs, and generally a few pools or large miners produce all blocks on those systems.
DPoS has been shown to help maintain decentralization, fend off regulatory interference, and has made cryptos like Steem into world players. This is also why you should take your witness votes on Steem very seriously; the people you vote for are possibly the ones who will be deciding/shaping the future of the network as well. Just like a democracy, the system will work better and be more decentralized/fair if more people participate and are informed.
Top DPoS Coins: https://coinmarketcap.com/view/pos/
In cryptography, a zero-knowledge proof or zero-knowledge protocol is a method by which one party (the prover) can prove to another party (the verifier) that they know a value x, without conveying any information apart from the fact that they know the value x. The essence of zero-knowledge proofs is that it is trivial to prove that one possesses knowledge of certain information by simply revealing it; the challenge is to prove such possession without revealing the information itself or any additional information.
It is proposed that ZKPs could be used to guarantee that transactions are valid despite the fact that information about the sender, the recipient and other transaction details remain hidden.
Source: https://en.wikipedia.org/wiki/Zero-knowledge_proof
