ETH (or Ether) was initially issued by Etheruem Foundation as major fundraising channel.
ERC-20 Standard is developed for blockchain projects to issue and transfer tokens (ERC-20 type) through Ethereum network. ERC-20 type tokens can be traded freely on Ethereum network.
However, ETH is NOT compatible with ERC-20 Standard.
To enable direct transactions between ETH and ERC-20 Tokens, "wrapping" and "unwrapping" actions are required for ETH - WETH conversion.
Wrapping: ETH -> WETH (ERC-20 compatible)
Unwrapping: WETH -> ETH (not compatible to ERC-20)