KNN algorithm assumes that similar things are near to each other. In KNN we select a number of neighbors of the things we're are interested in and based on the properties of its neighbor we predict something about the thing.
A Data Structure, as the name suggests, is a method to store data in a structured way so that it can be easily created, viewed, and managed. A one-liner definition may not be enough to explore Data Structures. So Let's take an example: