Plasma is one of the second layer scaling solution for Ethereum in development.
The Ethereum second layer is particularly concern with scalability.The scalability is the issue in Ethereum since the formation of Ethereum.
So lot of startups are working to tackle the scalability issue of Ethereum.Plasma is one of the solution for scaling and it is expected to fully deployed scaling solution after state channels.
Plasma focuses on creation of child chains.Plasma uses the combination of smart contract and cryptographic verification.This leads to make transaction more faster and reduces the gas fees too.
The transactions are carried on offchain by child chains(plasma chains) and then reported to main chain.This removes the higher gas fees,main chain burden and makes the main chain more secure.
The Ethereum main net and child chains are tied together by smart contracts called as 'root contracts'.This contracts contain the rules for guiding the child chains.
Similar to main chain plasma chains also have their own consensus mechanism.
Because of the root contracts plasma chain is tightly coupled with the main chain.Plasma is more secure than Sidechains.
Since in plasma chain if consensus mechanism stops creating the blocks then user can claim to main chain via block root.
In Plasma user has to wait to ensure security and sancity about transaction that finalized on mainchain after computed on plasma chain.
Plasma framework recommends waiting time of 7 days.This is why plasma is slow when it comes to sync details on mainchain.
The plasma structure is a tree like structure that can creates number of child chains and possesses the great potential for scaling.Plasma will help the Ethereum blockchain by taking operations off-chain.
Sidechains are another solution for scaling Ethereum(or any blockchain for that matter).
Sidechains have the tremendous potential to improve the capabilities of existing blockchains.
Sidechains are made purposefully to run as an another chain alongside to mainchain.And then they could talk to each other in way that make it possible to move assets between two chains.
Sidechains have their own consensus mechanism (may not be similar to mainchain).That gives the sole power to create blocks(like "Proof of Authority").
Sidechains are attached to main chain using two-way-peg.It is like locking(deposite) the asset you want to move to sidechain on mainchain and creating same asset on sidechain.
Meanwhile it is providing some proof of locked asset details to sidechain.And same for unlocking too.
When user wants to come back to main chain he has to lock that asset on sidechain first and the same asset automatically gets unlocked(withdrawn) in main chain.So we can do transaction on sidechains same as we do on main chain.
Since locking and unlocking assets means doing only two transaction(deposite and withdrawal) on main chain and there we go ,we get scalability!
So that's how sidechain works.
But unlike Plasma here user can't claim to mainchain if consensus mechanism fails.
But here sidechain is only secure as long as its own consensus mechanism not get compromised.
Because in sidechain it gives you sole power to create new blocks.So what if you altogether stop creating new blocks.
That's why sidechain transaction fees are cheaper than mainchain transactions.
In mainchain if you pay higher gas fee then you will get more security and it makes your transaction faster.Its depend on how much you pay.
We have both Plasma and Sidechains as potential scaling solutions for Ethereum but they do have their own limitations too.
Lot of developments are going on to improve the potential of both plasma and sidechains.
Thats it for now!