Ultrasurf is a freeware program designed to evade internet censorship that exists in some countries. In this post I'll explain how it works, and what Ultrasurf can and can't do. It's a standalone .exe file - so you can run it from a USB stick without installation. When it's running, you access sites through Ultrasurf's own servers. This can make browsing slower, but this seems to be the price to pay for avoiding online censorship and filtering.
With Ultrasurf you can bypass firewalls and access blocked sites. Websites will not be aware of your IP address, but because all traffic goes through the Ultrasurf, the company itself does collect data. Ultrasurf say they only keep traffic logs for short periods and to monitor performance, but basically you just have to trust their organization with your information! Ultrasurf has been used extensively in China and during the Arab Spring uprisings. It's main use is evading web censorship, firewalls and blocked sites. In terms of 'private browsing', while it offers more anonymity than normal browsing, this has never been Ultrasurf's primary concern.
The biggest criticisms of Ultrasurf came this year in a post called 'Ultrasurf: the definitive review' on the Tor blog. In this extensive report the author criticized many aspects of Ultrasurf, but from a journalistic point of view it's a report that is hard to swallow. Tor is a competing product to Ultrasurf, and both clearly have different ideologies about internet privacy: Tor is better suited to someone who just wants anonymity (although Tor isn't 100% safe either), whereas Ultrasurf is more about evading censorship, and actually getting access to restricted sites.
Utrasurf is great at what it does, and if you need to get around firewalls, it's probably your best option, even if it's not the anonymity haven that some people want it to be.