The most important characteristic of a totalitarian system is that it allows its citizens to have less input into the way they are governed and fewer individual liberties than any other system. This characteristic is the source of this system’s name. In this system, the government tries to control every aspect of human life as much as possible.
The best examples of totalitarian systems in relatively recent times have been Hitler’s Nazi Germany and communist countries such as the USSR under Stalin, China under Mao, and North Korea for its entire history. In these countries, the people have essentially not had any ability to influence their government. There have been no true political parties. Dissent has been suppressed, often in brutal ways. People have had very few freedoms. The Soviet government, for example, tried to destroy religion in the country. There was no independent media in these countries (and there is still none in North Korea). The state has typically tried to intervene even in such things as family relations.
The main characteristic of a totalitarian system, then, is that the government tries to control every aspect of human life and gives its people essentially no political or personal freedoms.