Depending on how you use your Wordpress install, you can end up with some duplicate content issues. The robots.txt file blocks web spiders from accessing parts of your site that are otherwise viewable by the public. We can use this file to block the spiders from crawling areas of our site where content may appear additional times. Here is a pretty standard layout that I use:

User-agent: *
Disallow: /cgi-bin
Disallow: /wp-admin
Disallow: /wp-includes
Disallow: /wp-content/plugins
Disallow: /wp-content/cache
Disallow: /wp-content/themes
Disallow: /*.php$
Disallow: /*.inc$
Disallow: /*.css$
Disallow: */feed/
Disallow: /tag/

User-agent: Mediapartners-Google
Allow: /

User-agent: Adsbot-Google
Allow: /

User-agent: Googlebot-Image
Allow: /

User-agent: Googlebot-Mobile
Allow: /

User-agent: ia_archiver-web.archive.org
Disallow: /

The line User-agent: specifies which bot that the following lines will affect. Using the * specifies that the following will apply to all bots.

This setup blocks all the sensitive parts of your wordpress install. You can add more lines depending on what else you want to hide from the spiders. You can use this file to deal with duplicate content issues.
Here are some resources on the robots.txt file:
http://en.wikipedia.org/wiki/Robots_Exclusion_Standard
http://www.seoconsultants.com/robots-text-file/