"In addition" is used as a sort of conjunction leading into a new sentence when talking about something else of the same nature. Think of it like a more formal way to combine sentences without using the word "and". ex: "I like dogs and cats." "I like dogs. In addition, I love cats." "In addition to" requires a noun afterwards, unlike "in addition". Using the same sentence above: ex: "I like dogs and cats." "In addition to dogs, I also love cats." The first way "In addition" typically requires the use of 2 sentences, where "in addition to" can be used with one sentence.

In addition is a adverb pharse whereas in additin to is a propositonal pharse. Grammatically this is how different in term of usage as the following examples: 1. In additon to those subjects, they also taught history and geography. 2.Dr. York is a professor of art. In additon, he is a critic of television.