PirateBrowser allows you to circumvent censorship that certain countries impose onto their citizens such as Iran, North Korea, United Kingdom, The Netherlands, Belgium, Finland, Denmark, Italy and Ireland.

Does it make me surf the net anonymously?

No, while it uses Tor network, which is designed for anonymous surfing, this browser is intended just to circumvent censorship --- to remove limits on accessing websites your government doesn't want you to know about.

Does this contain any viruses or trojans?

There have been no modifications to any of the packages used, no adware, trojans, toolbars, etc. This is simply a tool to help people get around censorship.