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 Tornetwork</a> is public.
Parental controls, Internet service providers, and countries with heavy censorship can identify and block connections to the Tornetwork that don't use Tor bridges.
Tor hides your location from destination servers, but it does not encrypt <i>all</i> your communication. The last relay of a Tor circuit, called the <i>exit node</i>, establishes the actual connection to the destination server. This last step can be unencrypted, for example, if you connect to a website using HTTP instead of HTTPS.
Observe your traffic. That is why <i>Tor Browser</i> and Tails include tools to encrypt the connection between the exit node and the destination server, whenever possible.
The Tornetwork has more than 6 000 relays. Organizations running Torrelays include universities like the MIT, activist groups like Riseup, nonprofits like Derechos Digitales, Internet hosting companies like Private Internet Access, and so on. The huge diversity of people and organizations running Torrelays makes it more secure and more sustainable.
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.