A comparative essay asks that you compare at least two (possibly more) items. These can be:
Although the assignment may say “compare,” the assumption is that you will consider both the similarities and differences; in other words, you will compare and contrast. [source]
How to write a comparative essay: River Valleys (30 minutes)
Even though this video is about Ancient History, the process that the teacher takes the students through is quite good.
Compare/Contrast Essay (10 minutes)
A quick guide on how to write a comparision/contrast essay. Visual demonstration.