Some instructors prefer that you only write about the differences between two things, while others want you to focus on explaining the similarities as well.  Either way, you'll need to make sure that your thesis statement reflects your instructor's expectations. For example, if I wanted to write about Social Networking sites, I'd need to write different thesis statements depending on my compare/contrast assignment.  

Sample thesis statement for contrast paper:  In terms of social networking sites, Facebook focuses on presenting your daily life to others, whereas MySpace allows you to focus more on demonstrating your personal style.

Sample thesis statement for compare/contrast paper:  While both Facebook and MySpace allow you to meet other users who have similar interests, only MySpace allows you to demonstrate your personal style.       

Are these 2 things similar and/or different, in at least one meaningful way?

If you want to write a successful compare/contrast essay, you'll need to avoid writing about really obvious differences and similarities.  For example:

• We all know that horses are larger than cats.
• We also know that basketball teams contain less players than football teams.

Tell us something we don't know (or might not notice)!

It would be better to write about how sensitive both horses and cats are to human needs and emotions.  You could also suggest that though both basketball and football require a lot of teamwork, basketball players are expected to be a lot more versatile than football players.

You don't have to be a genius to write an interesting compare/contrast essay--you just have to look at ordinary things in a new way!

Do I know enough about my topic to write an effective compare/contrast essay?

Unless you're being asked to do some research as part of your compare/contrast project, make sure that you choose 2 things that you feel comfortable discussing, at length.

Your instructor may ask for multiple similarities and differences--make sure you're prepared to write a well-developed, meaningful essay on a topic that you know well before you get started!   

Organizing Your Compare and Contrast Paper

There are two primary ways to organize your compare and contrast paper.

Chunking: placing all of the information for each individual subject in one place (chunk), and then using similarities as transitions.

Here’s a sample outline:

1. Jane is distinct because…
2. Jane is similar to Alice in these ways
3. Alice is distinct because…

Piecing: giving pieces of the information for each individual subject in each paragraph—arranging the information by topic rather than by subject.

Here’s a sample outline:   

1. Differences and Similarities in Jane and Alice’s appearances
2. Differences and Similarities in Jane and Alice’s backgrounds
3. Differences and Similarities in Jane and Alice’s interests

http://www.roanestate.edu/owl/Mac.html