TREZOR is an isolated environment for offline transaction signing and using a small display you can visually verify the transaction contents. That's why all operations using TREZOR are entirely safe. The ultimate in ease of use and super high levels of security. A Bitcoin safe if you like. Trezor keeps private keys on its device and signs transactions through hooking up via USB. So you will need a wallet such as Mycelium on your phone or desktop. Trezor lowers the risk of your private keys being discovered even if your PC is compromised by malware. Although not the most practical wallet system for hot wallets and being on the go, but definitely the gold standard in what it does.