PCollections are a core abstraction in the Apache Beam programming model, designed to represent a collection of data that can be processed in parallel. They can handle both bounded data (fixed-size datasets) and unbounded data (streams of data that can grow indefinitely), making them versatile for various data processing tasks. PCollections support various operations, such as transformations and aggregations, allowing users to manipulate data efficiently. They can be created from different sources, including files, databases, or real-time data streams, and can be processed using different runners that execute the data processing pipeline on various platforms.