ETH stands for Ether, the standard currency used to pay for transactions. You use ETH to pay for transactions on L1, and you use it to pay for transactions on OP Mainnet (L2). This is the reason you need to deposit ETH before you can issue any transactions. Some systems call ETH on OP Mainnet OETH, but that's the wrong terminology. It functions as ETH, and through the bridge it is directly convertible to ETH at a 1:1 rate. It is ETH.
ETH predates the ERC-20 standard, and does not comply with it. For example, you can give an allowance of an ERC-20 token, but not with ETH. To enable ERC-20 functionality with ETH, there's the wrapped ETH (WETH) contract. You can deposit ETH with this contract, and receive in return the same amount of WETH tokens which are ERC-20 compliant. Those tokens can be traded, and their new owner can get the contract to burn them and get back the ETH those tokens represent.
Note: Do NOT send WETH tokens back to the WETH contract. They will be stuck there, and that isn't the way to get them refunded back to you. To convert WETH back to ETH you need to use the WETH contract's withdraw function. The parameter, wad, is the number of wei to withdraw.
There could be multiple WETH contracts on a blockchain, but it is ideal to have just one. That way you don't get into a situation where an application recognizes one WETH type while the user has another. In OP Mainnet, the WETH contract is a predeploy, deployed in address 0x4200000000000000000000000000000000000006. It uses version 9 of the contract.