A devnet is a separate network for testing, like the Aggron testnet, except that it is for private usage.
A devnet can be created instantly, without having to wait for the network to synchronize since it initializes from the genesis block. A devnet can be configured, multiplied, paused, or deleted as needed. Since devnets reside on the developer's computer and require no consensus, by definition it is centralized. However, this makes the network extremely practical and efficient for development.
For obtaining funds on a devnet, there is no faucet. Instead, there is a well-known account that contains a very large amount of CKBytes which are created in the genesis block. A developer can use the provided private keys to access these CKBytes, which should be more than enough for any necessary testing.
To read more, check out the official documentation for Running a CKB Dev Blockchain