The Oasis SDK provides a modular framework for building runtimes (also called ParaTimes) on top of the Oasis Core Runtime Layer. It provides standard types and wire formats for transactions, events, queries, denominations, addresses, etc. For composability it separates functionalities into modules that can be combined together to form runtimes with the desired functionality.
The following are the two main components that allow you to easily build runtimes and the supporting frontend applications:
Runtime SDK handles the backend part, namely the runtime itself. It allows you to build the logic that will be replicated when running alongside an Oasis Core node in Rust.
Client SDK handles the connection of the backend part with the frontend by providing libraries in different languages that make it easy to generate transactions, look up emitted events and query the runtime.