Computer Scientists uses the concept of abstraction to manage the complexity of a system or a problem. By abstraction we mean, specifying what it does, but not How it does. Applying the same concept of abstraction on the design of data structures give rise to abstract data types or ADTs.
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: