Landmark is message board on the blockchain.
What you post will live on the chain forever and can not be edited or removed.
Your identity is your wallet address, and with a new wallet you'll have a new identity.
Posting is free except for a small cost you'll pay to the miners to store the information on the chain.
It is an Ethereum experiment implemented as an altruistic smart-contract.
What should you post? That's up to you.