stETH is the tokenized ETH staked into the ETH Beacon Chain sometimes called ETH 2.0. stETH earns ETH POS staking rewards. 1 stETH is fully backed by 1 ETH on the beacon chain this means 1 stETH = 1 ETH. Anyone can mint 1 stETH with 1 ETH using the dApp Lido.
stETH is sometimes described as being "pegged" to ETH but this is not correct. stETH is backed by 1 ETH similar to how 1 USDC is backed by 1 fiat dollar in a bank account; compare this to DAI which is not backed by fiat dollars in a bank account but instead its price is pegged to the dollar.
stETH ontract on Ethereum mainnet: 0xae7ab96520de3a18e5e111b5eaab095312d7fe84
What is wrapped stETH (wstETH)?
Your stETH balance is not constant - it changes daily to reflect your earned rewards. This is what's known as a rebasable token.
This rebase works across integrated DeFi platforms like Curve and Yearn, but other platforms like UniSwap, 1inch and SushiSwap are not designed for rebasable tokens. As these other DeFi protocols - e.g. Uniswap - require a constant balance mechanism for tokens, Lido let's you "wrap" your stETH into wstETH to keeps your balance of stETH fixed. Instead of updating daily, wstETH uses an underlying share system to reflect your earned staking rewards that is applied when you unwrap your wstETH.
Read more about wstETH here https://help.lido.fi/en/articles/5231836-what-is-wrapped-steth-wsteth
wstETH contract on Ethereum mainnet: 0x7f39c581f595b53c5cb19bd0b3f8da6c935e2ca0