Java NIO (New Input/Output) is a Java library introduced in version 1.4 that provides a more scalable and flexible way to handle input and output operations. Unlike the traditional Java IO, which is stream-based and blocking, NIO uses a non-blocking approach, allowing for more efficient data processing, especially in applications that require high performance, such as servers. NIO introduces key components like Buffers, Channels, and Selectors. Buffers are used to store data, Channels represent connections to data sources, and Selectors allow a single thread to manage multiple channels, making it easier to handle multiple connections simultaneously without creating a new thread for each one.