A wallet stores the public and private keys which can be used to spend or receive cryptocurrencies. There are various types of wallets: hot wallets, cold wallets, paper wallets and hardware wallets. Each have their own benefits in terms of security and accessibility.
Trezor is a hardware wallet, which stores the private keys on the device itself. The wallet can be controlled from the Trezor Wallet interface, but the device is required to send transactions. The private keys never leave your Trezor device, ensuring that there is no risk of losing funds even if the Trezor is exposed to a potentially infected computer.