In 1984, the Party exercises a form of control over Party members and much of the population (including to a lesser extent the Proles) that extends to all aspects of society, even to the ability to express thought. The most fundamental areas of control are in this realm. From the control and manipulation of language and knowledge stem all other forms of control.
Syme, when speaking to Winston about the wonders of Newspeak, discusses how it works. With Newspeak, the Party ultimately seeks to demolish the English language by the year 2050. Newspeak systematically roots out words that have ambiguous or duplicate meanings, leaving only those words that are necessary. By extension, the words that remain also limit the ability to express oneself. It is the difference between a ten-word vocabulary and and one hundred word vocabulary. In the latter case, it is much easier to express oneself more clearly and in a more individual way. With only ten words, there is much less freedom in the resulting attempts at self-expression. In a political sense, this will ultimately silence political dissent, not through violence but through more subversive means.
Winston's job in the Ministry of Truth is to "rectify" newspaper articles and other communications to eliminate any information that could place the Party and Big Brother in an unflattering light. This includes communication about past events and about current events. Doing this conspicuously alters the population's perception of those events. When taken together, the use of Newspeak and the policy of "rectification" serve to take away the ability of the population to think and question what they are told. They physically cannot express their thoughts and they only have information supplied by the Party.
The telescreens and the Thought Police round out the three major tools the Party uses to control (to take away the freedom of) the population. Each citizen (Party member) is subject to the neverending glare of the telescreens. Through facial expressions, the telescreens can make judgments on what a person is thinking. From this, the Thought Police can take those citizens away. This reality makes it impossible to show any emotions that can be interpreted as negative to the Party.
Newspeak, the telescreens and the Thought Police, and the policies of the Ministry of Truth each serve to take away the freedom of the population to express itself, to make decisions of their own accord, and, as they all draw from the same small linguistic and information pool, ultimately to think and act as individuals.