1. A method of selecting an advertisement, the method comprising: receiving a video; extracting key frames from the video through a key frame extraction process, wherein the key frames are distinctive and representative of the video's subject matter; identifying one or more descriptors within the key frames, wherein the one or more descriptors comprise sharp edges, colors, and large blocks of color; determining a subject-matter category for the video using an image classification algorithm that analyzes the descriptors to determine the subject matter, wherein the image classification algorithm is a machine learning algorithm that uses training videos that are pre-classified into the subject matter to learn what descriptors are common to the subject matter; receiving an advertisement and one or more keywords associated with the advertisement; classifying the advertisement into the subject-matter category using a support vector machine that analyzes the keywords received with the advertisement; generating a first plurality of subject matter categories by analyzing a group of keywords associated with advertisements; ranking the subject-matter categories based on commercial advertising demand that reflects the commercial value of the subject-matter categories; generating a second plurality of subject-matter categories comprising subject-mater categories in the first plurality of subject-matter categories that have a commercial advertising demand above a threshold value; determining that two related subject-matter categories within the second plurality of subject-matter categories are too similar for the image classification algorithm to reliably distinguish between the two related subject-matter categories; as a result, combining the two related subject-matter categories in to a new category that encompasses the two related subject-matter categories, thereby causing subject-matter categories within the second plurality of subject-matter categories to be distinguishable and have a commercial advertising demand above the threshold; and storing the advertisement.