At the Trustlines Developer Docs site you can find all kind of technical documentation about Trustlines as well as code related documentation.
The Trustlines Blockchain is a minimal viable Proof-of-Stake (mPoS) Ethereum sidechain.
The clientlib is a JS-library which makes it easy to build applications on top of the Trustlines Protocol. It provides a high level API to enable applications to interact with the smart contract system on the blockchain via the relay servers.
The Relay Servers are an optional bridge between client apps and the Trustlines blockchain. They offer services which are not feasible to be implemented on-chain or within the client apps. Importantly, Relay servers calculate optimal paths and relay transactions.
The smart contract system is a collection of solidity contracts deployed on the Trustlines Blockchain. Transfers within the Trustlines Network are executed by the smart contracts. All trustlines (i.e. credit lines and balances between users) are notarized on the Trustlines Blockchain. Furthermore, the smart contracts enforce the rules determining how trustlines can be created, used and updated.