An important distinction to be made about sidechains that needs to be understood is that sidechains themselves help to fuel innovation through experimentation. Rather than providing scalability directly, they allow for trivial experimentation on sidechains with various scalability mechanisms. Using sidechains, one can avoid the problems of initial distribution, market volatility, and barriers to entry when experimenting with altcoins due to the inherent derivation of their scarcity and supply from Bitcoin. That being said, each sidechain is independent and flexible to tool around with various features.

Alpha functions as a sidechain to Bitcoins testnet. The peg mechanism currently works through a centralized protocol adapter, as stated in the sidechains whitepaper. An auditable federation of signers manages Testnet coins transferred to the sidechain. The federation is also relied upon to produce blocks through the signed blocks element. This creates the possibility of exploring the possibilities of the new chain using different security trade-offs.
This approach isn’t fool-proof, but it’s not by mistake that the system looks the way it does today (that’s my history degree talking). Despite best technical efforts, human problems remain within the realm of probability. From “…blame cannot be easily assigned: not even the most sophisticated economists of the era could accurately predict disaster, let alone guard against it. The effects of a public herd mentality at the time of the [insert catastrophe here] are depicted, all too recognizably, as unstoppable.”
Using Rootstock as an example, in order to transfer assets from one chain to the other a user on the parent first has to send their coins to a special output address where they will consequently become locked and un-spendable. Once the transaction is completed, SPV then confirms it across the chains and after waiting out a contest period, which is just a secondary method to help prevent double spending, the equivalent amount will be credited and spendable on the Sidechain and vice versa.

Sidechains solve a lot of problems, but at what cost? The introduction of sidechains makes things even more complex and much harder to understand for those who are not actively involved in the blockchain space. This also divides assets, no more “one chain, one asset” adage, which further complicates things. And on a network level there are multiple independent unsynchronised blockchains interacting with each other.
Consider a proof-of-existence application, where you want to authenticate your document in the Ethereum (for example) network, but you do not need your document to be online. So, you will store the hash generated from your document in the blockchain, but the document itself will be in your local machine, out of any blockchain-related structured, being off-chain.
@mowliv I think a good way to think about it is by looking at our economy. The Federal Reserve prints US dollars for the US Government (the main blockchain) to boost the US economy. However, US dollars can be exported to other countries (a side chain) that could have a completely independent economy but still use a currency backed by the US government. – Olshansk May 30 '17 at 0:56

Another technology that could see more widespread use in the coming years is side chains. A side chain is defined for one specific use case. There can be multiple side chains where different tasks are distributed accordingly for improving the efficiency of processing. Maybe one application needs to optimize for high speeds and another needs to optimize for large computations. In any case, side chains can be used to handle commercial blockchain usage. CryptoKitties would have greatly benefitted from an optimized high-speed side chain. At one point, they jammed up the Ethereum blockchain with 25% of all transactions coming from their application.

Lisk es una plataforma open source en la que se pueden desarrollar y ejecutar smart contracts en forma de aplicaciones descentralizadas o DAPPS multiplataforma. Éstas, y como uno de los puntos fuertes de Lisk, son desarrolladas con, posiblemente, el lenguaje de programación más famoso y usado, Javascript. Aunque con un enfoque genérico, ya han empezado a aparecer algunas soluciones e interés en sectores concretos, como es el caso del Internet de las cosas que, junto a Chain of Things, están empezando a explotar.
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State of the art public Blockchain protocols based on Proof of Work (PoW) consensus algorithms are open source and not permissioned. Anyone can participate, without permission. (1) Anyone can download the code and start running a public node on their local device, validating transactions in the network, thus participating in the consensus process – the process for determining what blocks get added to the chain and what the current state is. (2) Anyone in the world can send transactions through the network and expect to see them included in the blockchain if they are valid. (3) Anyone can read transaction on the public block explorer. Transactions are transparent, but anonymous/pseudonumous.
Jump up ^ Epstein, Jim (6 May 2016). "Is Blockchain Technology a Trojan Horse Behind Wall Street's Walled Garden?". Reason. Archived from the original on 8 July 2016. Retrieved 29 June 2016. mainstream misgivings about working with a system that's open for anyone to use. Many banks are partnering with companies building so-called private blockchains that mimic some aspects of Bitcoin's architecture except they're designed to be closed off and accessible only to chosen parties. ... [but some believe] that open and permission-less blockchains will ultimately prevail even in the banking sector simply because they're more efficient.
In this article, I will intent to do a public vs private (permissioned) blockchain comparison. This will include an examination of what exactly the roles of these two types of blockchain really are and why big businesses should quickly move to adopt them. This analysis will look at why private blockchains are better suited to big business use when compared to public ones.
A side-chain is a separate block-chain that runs parallel to the main chain, for example the Bitcoin network, and is attached to the main chain through a simple two-way peg, or special 'address'. A user sends coins to this special address and this amount is effectively 'locked' out from use on the main chain and available on the side chain. This currency is released back to the main chain once its been proven that the side chain is no longer using it.
The ethereum-based app builder has a dedicated team of experts looking at all varieties of fiat cash on distributed ledgers, and it's working with UnionBank of the Philippines to create a low-cost tokenized fiat solution for rural banking. In time, this could be extended to cover a larger network of banks and perhaps even the central bank, ConsenSys says.