The sites that you visit can know that you are using Tor, because the <a href="https://metrics.torproject.org/exonerator.html">list of exit nodes of the Tor network</a> is public.
Parental controls, Internet service providers, and countries with heavy censorship can identify and block connections to the Tor network that don't use Tor bridges.
Pretend to be the destination server, a technique known as <i>machine-in-the-middle</i> attack (MitM). That is why you should pay even more attention to the security warnings in <i>Tor Browser</i>. If you get such a warning, use the [[New Identity|doc/anonymous_internet/Tor_Browser#new-identity]] feature of <i>Tor Browser</i> to change exit node.
Tor exit nodes have been used in the past to collect sensitive information from unencrypted connections. Malicious exit nodes are regularly identified and removed from the Tor network. For an example, see <a href="https://arstechnica.com/information-technology/2007/09/security-expert-used-tor-to-collect-government-e-mail-passwords/">Ars Technica: Security expert used Tor to collect government e-mail passwords</a>.
A powerful adversary, who could analyze the timing and shape of the traffic entering and exiting the Tor network, might be able to deanonymize Tor users.These attacks are called <i>end-to-end correlation</i> attacks, because the attacker has to observe both ends of a Tor circuit at the same time.
No anonymity network used for rapid connections, like browsing the web or instant messaging, can protect 100% from end-to-end correlation attacks. In this case, VPNs (Virtual Private Networks) are less secure than Tor, because they do not use 3 independent relays.