A dataset is a collection of related data, however what constitutes a dataset is not clearly demarcated. One could consider all the data associated with a research project, regardless of the type of data, a single dataset. But a research project will often collect disparate types of data (e.g., physiological, survey, and genomic), and one could consider data collected through each method a separate dataset. Different people may argue about different ways to define a dataset, and all the perspectives could be legitimate.
There are many searches for available datasets, including Kaggle: https://www.kaggle.com/datasets
Elon University created a video that provides a basic description of a dataset: https://www.youtube.com/watch?v=eHoZnQmSEzs