I plan to develop a easy application in blockchain,implement leave comments in blockchain. Use OP_RETURN,Information recorded in the blockchain.

See bitcoin's blockchain

https://blockchain.info/tx/e9b30933e42b8d8273679b5e71ea33302a0ea5e801313413ec0c5b160dd37cb6?show_adv=true

OP_RETURN 30205468697320697320612074657374206465636c61726174696f6e2062792074686520686561 
(decoded) j'0 This is a test declaration by the hea

This is a proof of existence.

How to use it

See News http://www.coindesk.com/monegraph-uses-block-chain-verify-digital-assets/

What is OP_RETURN?

OP_RETURN is a custom transaction locking script that can store data, but nothing beyond that, the execution of the script will be terminated immediately when bitcoin nodes encounter UTXOs that contain it - this means that you wonâ€™t be able to spend that transaction output.

When using OP_RETURN properly you will also avoid destroying coins.

Limitations

OP_RETURN scripts are limited to 40 bytes, this is enough for a SHA-256 checksum (32 bytes) with 8 bytes of prefix or for a shortened URL
There can be only one OP_RETURN per transaction