Tor is an anonymity network using onion routing to make it difficult for adversaries to track users of the network. Tor directs the traffic to three relays (guard, middle, exit) before sending it to the server. This way, the server can't accurately know the location of the user via IP address.
Users commonly use the official Tor Browser, a Firefox fork designed to connect to Tor. Although any browser can be customized to use Tor, the Tor Project, maintainer of the Tor software, recommends to use the Tor Browser whether a novice or experienced one, in order not to compromise security and privacy.
Tor is used both for legal (like whistleblowing or simply for increased privacy) and illegal purposes (like dangerous drugs). The bad sides shouldn't be a reason why not to use Tor, because if it is, then cars, guns, and knives should be banned because they can cause harm. Tor is important for activism, journalism, the military and police, and even normal people who just care about their privacy for them and their family (especially when Edward Snowden revealed the surveillance programs of the National Security Agency). Without Tor, the good citizens have no defense unlike criminals who always have anonymity tools other than Tor.