Seastar is a C++ asynchronous programming framework that allows for multi-domain async programming across networking, storage I/O, and multi-core communications. It uses an event-driven model where each logical core runs a task scheduler independently. Logical cores communicate through queues. Seastar is applicable for workloads with high I/O to compute ratios, high concurrency needs, and distributed applications. It provides futures/promises abstractions and rich APIs for tasks like HTTP servers, RPC, and distributed databases.