The Principles of Design are concepts that refer to the relationships of the Elements of Art to each other and to the total composition. They guide how we arrange those visible elements to make effective compositions. They include: pattern, contrast, emphasis, balance, scale, harmony, rhythm/movement, unity, and variety. They are defined here. Not all of the principles of design are needed in each work of art but they should be considered when creating.
What do mediums (media) do artists use to create artwork? - Per Wikipedia, art media is the material and tools used by an artist, composer or designer to create a work of art. They use the Principles of Design to compose their artistic intention and convey their message to the audience.
What does technique mean? Technique is the method with which an artist, writer, performer, athlete, or other producer employs technical skills or materials to achieve a finished product or endeavor. For example, if you are are using a brush the technique is most likely to be "painting" while when creating computer vector animations the technique is most likely to be that of traditional animation (image by image development).