Blockchain is one the most important part of the cryptocurrency. This is the open ledger where you can see the transaction publicly or privately. Basically it is been cast to the network from the computer who has its wallet installed. Transaction are posted every blocks in the blockchain whether it was received or sent.
Every time you make a transaction, it will be posted to the blockchain network and can be see in a block explorer site for example in bitcoin is https://blockchain.info/ or https://blockexplorer.com.
Other coins like ethereum has his own custom blockchain that allow also token details and transaction aside from etherium itself. Example block explorer for etherium is https://etherscan.io/
The Risk
Transaction in cryptocurrency is irreversible so make sure that you check them when ever you spend your coin. Always check if the address is the proper coin address for the coin. Every coin has their own unique coin addresses, for example you can not send litecoin to a bitcoin address and bitcoin to etherium address and vice versa, their protocol doesn't allow it.
Be careful to whom you transact with or to whom you send your coin. Always take care your passwords, passphrase, and pass keys to avoid loss and always back-up your wallet especially when you are using a PC wallet.
Though you can see the transaction in a blockchain it doesn't allow you to edit or reverse it back. You can only ask to whom you have transact with to send it back if it was mistakenly send. Coin sent in a wrong coin addresses can not be return.
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