For example, an ERC-20 Token acts just like the ETH, meaning that 1 Token is and will always be equal to all the other Tokens.
The most used implementation of the ERC20 standard is the one from OpenZeppelin. Most ERC20 tokens have 18 decimals, just as ether, that means each token can be divided up to 10-18.
ERC20 tokens are used, on EVM-compatible blockchains, to represent property of some asset (which can come from the real world or from the blockchian). They're used for example to port other cryptocurrencies on external blockchains (ex WBTC). They're used for governance on many protocols or even for liquidity mining. They're also used to represent other assets (see Stablecoin) like currencies, stocks, commodities etc...
- transfer tokens from one account to another
- get the current token balance of an account
- get the total supply of the token available on the network
- approve whether an amount of token from an account can be spent by a third-party account
(Examples of ERC20 Tokens: $USDC, $LDO, $USDT, etc)