Entering Tiananmen Square requires you to pass through a security checkpoint beneath ground before emerging into a large concrete space flanked by Tiananmen Tower, Monument to the People's Heroes, Great Hall of the People, Chairman Mao Zedong Memorial Hall.
The Forbidden City is marked by Mao's portrait to the north. Combined with Tiananmen Square, these two sights draw thousands of visitors everyday. Admittedly, they are slightly different types of tourists, as the flag raising and lowering ceremony in the center of the square is more heavily skewed toward domestic tourists than international tourists.
Tiananmen Square is known more internationally for the 1989 student-led protests that ended with soldiers and tanks rolling into the square and hundreds of protestors 'disappearing'. The June Fourth Incident, as it's known in China, is still censored heavily and is not discussed freely out of fear.
Because of its importance to modern day China, Tiananmen Square is a must stop for any tourist passing through Beijing.