Tor (originally short for The Onion Router) began life as a US Navy project for anonymized online activity, but is now in used by a wide range of people including those in the military, journalists, bloggers, activists (and yes, sometimes, criminals). Tor makes communications harder to trace through traffic analysis by routing your internet activity through a series of network nodes, with each node ignorant of the whole route from beginning to end. There are tradeoffs though, mainly in terms of speed, as the extra routing will slow down the transmission. The Tor Browser Bundle is a package that contains an easy to use and install portable browser and the required software to connect that browser to the Tor network. The whole thing allows for anonymized online activity. It's not perfect, as Tor itself will point out in their website, but combined with smart browsing habits, it can reduce your exposure online.