The best answer is to be found in the sidechains whitepaper - it's extremely clearly answered in there:
https://blockstream.com/sidechains.pdf
Topic: What is a sidechain ? (Read 570 times)
Best answer I can give you is that a sidechain is designed to run parallel to the main Blockchain and may exchange data with the main Blockchain periodically. In a lot of cases, the sidechain can send data or receive data from the main Blockchain, but can't do both. If you've ever heard of Bitcoins being "burned," that means those Bitcoins are being locked up in an unspendable form by transferring them to a "one-way" sidechain that can't transfer them back. Sidechains are often used for experimental purposes -- for instance, you might transfer a few Satoshis from the main Bitcoin Blockchain to a sidechain to test a new feature you're working on.
Thanks for your answer commandrix!
Does the creation of a sidechain requires creating a new crypto-currency ? Or can you interact between the main blockchain and the sidechain with the same crypto-currency ?
Sidechains don't really require a "new" cryptocurrency. The difference between a sidechain and a whole new main Blockchain (which is necessary for a new cryptocurrency that operates independently from Bitcoin) is that a new main Blockchain wouldn't ever be able to exchange data with the original Blockchain, which is why forking a chain is not something to be taken lightly. Sidechains just require the ability to run parallel to the main Blockchain and sometimes exchange data.
Can we say that Sidechains and Sidecoins are the same thing ?
Best answer I can give you is that a sidechain is designed to run parallel to the main Blockchain and may exchange data with the main Blockchain periodically. In a lot of cases, the sidechain can send data or receive data from the main Blockchain, but can't do both. If you've ever heard of Bitcoins being "burned," that means those Bitcoins are being locked up in an unspendable form by transferring them to a "one-way" sidechain that can't transfer them back. Sidechains are often used for experimental purposes -- for instance, you might transfer a few Satoshis from the main Bitcoin Blockchain to a sidechain to test a new feature you're working on.
Thanks for your answer commandrix!
Does the creation of a sidechain requires creating a new crypto-currency ? Or can you interact between the main blockchain and the sidechain with the same crypto-currency ?
Sidechains don't really require a "new" cryptocurrency. The difference between a sidechain and a whole new main Blockchain (which is necessary for a new cryptocurrency that operates independently from Bitcoin) is that a new main Blockchain wouldn't ever be able to exchange data with the original Blockchain, which is why forking a chain is not something to be taken lightly. Sidechains just require the ability to run parallel to the main Blockchain and sometimes exchange data.
Best answer I can give you is that a sidechain is designed to run parallel to the main Blockchain and may exchange data with the main Blockchain periodically. In a lot of cases, the sidechain can send data or receive data from the main Blockchain, but can't do both. If you've ever heard of Bitcoins being "burned," that means those Bitcoins are being locked up in an unspendable form by transferring them to a "one-way" sidechain that can't transfer them back. Sidechains are often used for experimental purposes -- for instance, you might transfer a few Satoshis from the main Bitcoin Blockchain to a sidechain to test a new feature you're working on.
Thanks for your answer commandrix!
Does the creation of a sidechain requires creating a new crypto-currency ? Or can you interact between the main blockchain and the sidechain with the same crypto-currency ?
@commandrix, nice explanation 
In ethereum you don't need sidechains. Everything is done on the ethereum blockchain.
Best answer I can give you is that a sidechain is designed to run parallel to the main Blockchain and may exchange data with the main Blockchain periodically. In a lot of cases, the sidechain can send data or receive data from the main Blockchain, but can't do both. If you've ever heard of Bitcoins being "burned," that means those Bitcoins are being locked up in an unspendable form by transferring them to a "one-way" sidechain that can't transfer them back. Sidechains are often used for experimental purposes -- for instance, you might transfer a few Satoshis from the main Bitcoin Blockchain to a sidechain to test a new feature you're working on.
Can somebody answers clearly to this question ?
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