The Streams API, introduced in Java 8, is a declarative way to process sequences of data. Instead of writing loops that say how to iterate, you describe what you want as a pipeline of operations, filter these, map them to that, collect the result, and the library handles the mechanics. A stream is not a data structure that stores elements; it is a view over a source (a collection, an array, a generator) that carries elements through a chain of operations.
A stream pipeline has three parts: a source, zero or more intermediate operations (like filter and map) that are lazy and return a new stream, and exactly one terminal operation (like collect, forEach, or reduce) that triggers processing and produces a result. The lazy, build-then-run design enables important optimisations and a clean, composable style.
Understanding streams matters because they have become the idiomatic way to transform and aggregate data in Java, replacing verbose loops with readable pipelines that express intent directly. Grasping the source-intermediate-terminal structure, the laziness of intermediate operations, and the core operations, filter, map, reduce, and collect, equips you to write concise, declarative data processing that is often clearer and sometimes faster than the equivalent loops.