A dataset refers to a structured collection of data that is used for various purposes such as analysis, training of machine learning models, or conducting research. It typically comes in tabular form, where rows represent data entries and columns represent features or attributes of the data. For instance, in a dataset used to train a facial recognition system, rows might represent individual faces and columns data about the face like pixels, age, and so on. Datasets are crucial as they provide the foundational input that algorithms learn from and make predictions or insights. They can vary in size from a few entries to billions of records, and quality is crucial to ensure the accuracy of models.