In many cases, it is important to achieve anonymity on the Internet. Currently, there are many solutions to stay anonymous on the Internet. One of the most popular solutions is the Tor (The Onion Router). Tor has been hot topic for several years and most probably will stay that in years to come. Tor uses onion routing to prevent others to trace back the communication to its source, thus achieving the anonymity of the source. Tor can be used for many purposes that come from the anonymity, like avoiding the Internet censorship, whistleblowing, but also for illegal activities, too. Since Tor has gained great popularity, there is intensive ongoing research for Tor’s weaknesses. The focus of this presentation is to introduce and explain the main principles of the Tor - how Tor achieves the anonymity of the source. The potential applications of Tor and the weaknesses of Tor will be discussed as well.