Characterization is the way in which an author uses description, dialogue, inner motivation, and action in order to shape a character's personality.
Comparison is a wide term for rhetorical devices such as similes and metaphors. Remember that a metaphor only states one part of the comparison and leaves the reader to infer the other through the choice of words involved. For example, in the sentence "The soldiers stormed the city," you are led to think that the soldiers fell upon the city in the same way a storm would have hit it.
Simile is a kind of comparison in which two things or ideas that might seem unrelated are contrasted and joined by "like" or "as" to emphasize the ultimate meaning that the writer wishes to convey. An example of this would be "An emerald is as green as grass/A ruby as red as blood;/a sapphire shines as blue as heaven;/a flint lies in the mud." ("Flint," by Christina Georgina Rosetti.) The first three lines are similes that enhance the impact of the fourth line, which stands in opposition to the romantic images offered before it.