Sketching is typically a drawing that is not always a finished work. It might just be a record of something that the person sketching it sees. Sketching can also just be a rough drawing or painting where the artist notes down their main idea for a work that will eventually have more detail and precision.
There are about four most popular styles of sketching: interior sketching, fashion sketching, industrial sketching, and travel sketching. For interior sketching, it is important to understand perspective and to train your eye to judge scales and proportion. It’s good to try and finish the drawings in different time frames where you can get different results. For fashion sketching, it’s a good idea to have some sort of idea for human proportions of the face and body. The goal of fashion sketching is to be able to get a certain design idea in the best possible way. For industrial sketching, you would need to be able to draw objects in 3D to be able to clearly show the structural design and volume of the object. For travel sketching, an important part of this skill is to be able to convey the feel of the place and to capture a fitting moment at the same time to be able to make a correct scale and proportion of the object that is being sketched. Typically, you will also need to be quick while you sketch because normally you would be traveling on the road when you don’t have hours to work on it to get the correct thing that you are looking for in the sketch.