ERC20 tokens are custom user cryptocurrencies created on Ethereum based on successful ERC20 Token Standard. Currently, there is over 65,000 of ERC20 tokens in existence of which the most have no market value, see their full list.
Advanced Ethereum users can create a new ERC20 token fast and conveniently and have the advantage of the new token support in Ethereum wallets right from the start. This is also why ERC20 tokens have been widely used in ICOs and crowdsales.
How to use ERC20 tokens with Trezor
Even though ERC20 tokens are currently not integrated in Trezor Wallet (although work is in progress to change that soon), they can be safely used with certain third-party wallets and services, with the seed and private keys fully protected by the Trezor device. These include MyEtherWallet and MyCrypto. For more information about other third-party services, see Apps.
Support in Trezor devices
Trezor firmware supports all ERC20 tokens since firmware version 1.5.0 (for more information, see this blog article).