On February 28, the Ethereum community received the hard fork it had awaited since a security bug delayed its implementation in mid January: Constantinople. But what is Constantinople exactly, and what does St. Petersburg have to do with it?
Below we discuss upgrades, Constantinople’s security issues, EIPs, St. Petersburg’s arrival, the “Difficulty Bomb,” and the reason behind it all; Ethererum’s quest to transition from proof-of-work to proof-of-stake.
So what we need to do with this information?
If I hold ETH?People who own ETH coin do not need to take any special measures unless the exchange, web, mobile, or hard wallet they use tells them to do something. As Vitalik Buterin explained, there’s no need to “dump your non-Constantinople coins”.
If I mine or run an Ethereum node?When an upgrade happens, certain changes to the Ethereum protocol are written into the various Ethereum clients; if you run a node or mine ETH coins, you need to make sure you update your client, in order to sync with these changes. If you fail to update, your client will sync to the pre-network upgrade blockchain; you will be connected to an incompatible chain, unable to send or receive ETH. This list of links was provided by the official Ethereum blog:
Latest Geth client (v1.8.23)
Latest Parity client (v2.2.10-stable)
Latest Harmony client (v2.3 Build 74)
Latest Pantheon client (v0.9.1)
Latest EthereumJS VM client (v2.6.0)
Latest version of Nethermind client (v0.9.4)
Latest version of Ethereum Wallet/Mist (v0.11.1)
Why deploy two upgrades at once?
As you may remember, the Ethereum Constantinople upgrade was due to launch back on January 16, and was delayed because of security issues. In order to fix these issues on the Ethereum test networks, St. Petersburg was deployed at the same block height.
source:
https://coin360.com/blog/constantinople-and-st-petersburg-ethereum-forks-a-crash-course 
