Trezor is a hardware wallet providing advanced security for handling Bitcoin and other cryptocurrencies private keys.
Unlike traditional cold storage methods (offline storage or paper wallets), Trezor makes secure payments without exposing your private keys to a potentially compromised computer. See Security philosophy for more info.
Trezor is a small single-purpose computer. It is designed to protect your private keys from possible online and offline risks. To do this, Trezor keeps the private keys away from the Internet and confirms the transactions in-device. That way, you can always check the screen for the right address, amount and transaction fee before sending your bitcoins.
In layman’s terms, you can connect Trezor to an infected computer and still be able to have 100% control over the funds in your device.
This is great, then how to develop applications integrating Trezor? Do I need to buy Trezor for integration development?
Luckily, Trezor team released a great emulator which is based on same source code of production version of Trezor devices.
https://github.com/trezor/trezor-core
Follow all the steps, then you can see running instance of Trezor on your computer screen.