Imagine there is a Bitcoin-like system out there that you’d like to use. Perhaps it’s litecoin or ethereum or perhaps it’s something brand new.   Maybe it has a faster block confirmation interval and a richer scripting language. It doesn’t matter.   The point is: you’d like to use it but would rather not have to go through the risk and effort of buying the native tokens for that platform. You have Bitcoins already. Why can’t you use them?
A sidechain is a separate blockchain that is attached to its parent blockchain using a two-way peg. The two-way peg enables interchangeability of assets at a predetermined rate between the parent blockchain and the sidechain. The original blockchain is usually referred to as the ‘main chain’ and all additional blockchains are referred to as ‘sidechains’. The blockchain platform Ardor refers to its sidechains as ‘childchains’.
“We believe that public blockchains with censorship resistance have the potential to disrupt society, when private blockchains are merely a cost-efficiency tool for banking back offices. One can measure its potential in trillions of dollars, the other in billions. But as they are totally orthogonal, both can coexist in the same time, and therefore there is no need to oppose them as we can often see it.” 
The main point of a side-chain is to allow cryptocurrency networks to scale and interact with one-another. For example alt-coins and Bitcoin run on separate chains, however side chains allow for these separate currencies to be transferred through these two-way 'portal's or interfaces via a fixed conversion amount. Added benefits of side-chains are different asset classes like,stocks, bonds etc being integrated through a converted price onto the main chain, along with additional functionality like smart contracts,unique D-Apps, micro-payments and security updates that can be later incorporated into the primary network from these side-chains.
Sidechains offer a way for new, more radical settings and technologies to be implemented without affecting the main chain. This ensures that the main chain is as secure as possible whilst providing the freedom to explore options which would never be considered for use on the main chain. Sidechains should be quite powerful as they provide cases like anonymity, transparency, confirmation times and turing complete options like rootstock all whilst utilizing bitcoins rather than relying on the hashing power (security) of some far less secure alt coin. That being said… there is quite some controvery regarding blockstream’s funding of most of the core development team and their inflexiblity regarding the max blocksize. This inflexibility has directly contributed to the success of ethereum and it remains to be seen whether the dream of bitcoin maximalism will survive long enough for sidechains with all of the promised functionality to be rolled out. I am skeptical.

Forbes reports that blockchain and biometric eyeball scanning technologies underpin the systems that support food distribution in the Syrian refugee crisis. While there are many further uses of blockchain, at the core of its business functionality is the creation of transparent, stacking “ledgers” of information. This is where private blockchain can prove extremely useful.
Given all of this, it may seem like private blockchains are unquestionably a better choice for institutions. However, even in an institutional context, public blockchains still have a lot of value, and in fact this value lies to a substantial degree in the philosophical virtues that advocates of public blockchains have been promoting all along, among the chief of which are freedom, neutrality and openness. The advantages of public blockchains generally fall into two major categories:

Permissioned blockchains use an access control layer to govern who has access to the network.[46] In contrast to public blockchain networks, validators on private blockchain networks are vetted by the network owner. They do not rely on anonymous nodes to validate transactions nor do they benefit from the network effect.[47][better source needed] Permissioned blockchains can also go by the name of 'consortium' or 'hybrid' blockchains.[48]
A Sidechain, in simplest terms, is just a separate blockchain but is attached to the parent through the use of a two-way peg which allows for assets to be interchangeable and moved across the chain at a fixed deterministic exchange rate. This two-way peg works by utilizing simple payment verification or SPV as it's otherwise known. To show and prove ownership of the assets on the parent chain.
Nodes can be trusted to be very well-connected, and faults can quickly be fixed by manual intervention, allowing the use of consensus algorithms which offer finality after much shorter block times. Improvements in public blockchain technology, such as Ethereum 1.0's uncle concept and later proof of stake, can bring public blockchains much closer to the "instant confirmation" ideal (eg. offering total finality after 15 seconds, rather than 99.9999% finality after two hours as does Bitcoin), but even still private blockchains will always be faster and the latency difference will never disappear as unfortunately the speed of light does not increase by 2x every two years by Moore's law.
When you send Bitcoins somewhere, you lay down the challenge for the next owner. Usually, you’ll simply specify that they need to know the public and private keypair that correspond to the Bitcoin address the coins were sent to. But it can be more complicated than that. In the general case, you don’t even know who the next owner is… it’s just whoever can satisfy the condition.
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My take is that the Bitcoin architecture is a solution to the problem of how to maintain consensus about a ledger when the participants are unknown and many of them are adversarial (I know this is loose language… computer scientists working in the consensus space are more precise but I think this captures the essence…. i.e. we’re explicitly in a world where there is no “leader” and no identities for those providing the consensus services).
Things get a bit more interesting when you replace the single custodian with a federation of notaries by way of a multisignature address. In this model, a federation of entities must sign-off on movements to and from the sidechain, so more parties must be compromised for a failure situation to unfold where the bitcoins frozen on the main chain are stolen.

Confidential Transactions — At present, all Bitcoin transactions are completely public, albeit pseudonymous. Confidential Transactions, as the name implies, conceal the amount being transferred to all except the sender, the recipient, and others they designate. The resulting transaction size is significantly larger, but includes a sizable “memo” field that can be used to store transaction or other metadata, and is still smaller than eg Zerocoin.(Note that this isn’t as confidential as Zerocash, which conceals both the amount and the participants involved in any transaction, through the mighty near-magic of zk-Snarks. Mind you, Zerocash would require an esoteric invocation ritual to initiate its network. No, really. But that’s a subject for a separate post.)
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"I see quite a few use cases for private blockchains, and they definitely have their place. Traditional institutions won't switch to a completely public blockchain from one day to the other. A private blockchain is a great first step towards a more cryptographic future. The biggest advantages of private blockchains in comparison to centralized databases are the cryptographic auditing and known identities. Nobody can tamper with the data, and mistakes can be traced back. In comparison to a public blockchain it is much faster, cheaper and respects the company's privacy. As a conclusion, it's better to rely on a private blockchain than no cryptographic system at all. It has merits and pushes the blockchain terminology into the corporate world, making truly public blockchains a bit more likely for the future." 
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