Anti-communism is opposition to communist ideology, organization, or government, on either a theoretical or practical level. In some of the earlier 19th century usages anti-communism referred to people opposed to the growth of independent, self-reliant and often religious communities such as the Community Oneida] and  communities. After the October Revolution the first critics of communism were inspired by a conservative point of view, but with the raising of Stalinism many exponents of the left, some ex-communists included, opposed the Soviet Union for its violations of human rights. For much of the period between 1950 and 1991 anticommunism was one of the major components of the containment policy of the United States.
For this last reason the word is sometimes used with a negative meaning to define an opposition to communism schematic and excessive, which doesn't take in consideration the differences between various communist regimes and movements and it is instrumentally used as a political weapon in the clash between West and East. This bias against anticommunism is even due to the opportunistic use of anticommunism made by some authoritarian regimes to persecute dissidents of any political colour.
In fact, the reasons because of several people opposed communism can be very different and sometimes in contrast between them. Conservative and liberal critics of communism often opposes socialism in general or Marxism in general. They are supporters of capitalism and they see communism as a doctrine based on radically wrong arguments. They believe that capitalism gives economic freedom, and regard the lack of property rights under communism as taking away fundamental human rights. Communists respond to this by arguing that the presence of property rights in capitalism takes away other, more important human rights, alluding to the disparities of wealth that all capitalist nations possess, to varying degrees.
Other people oppose communism due to contradictions or errors within communist theory and gaps between communist theory and practice. Many anti-communists feel that the theory is less objectionable than its adherents' actions in power. Democratic socialists as George Orwell or Bertrand Russell and anarchist theorists see communism as a doctrine whose aims are 'noble' in theory but that uses wrong means to attain them. A main critique of communism concerns the lack of individual freedom and democracy in communist states, democracy which is not denied by the communist theory itself (although interpretated in a very different way than that of liberal democracy).
Some anti-communists refer to both Communism and fascism as totalitarianism, seeing a certain degree of similarity between the actions of Communist and fascist governments. It should also be noted that many communists, particularly Trotskyists, use these similarities to argue that those self-proclaimed Communist regimes (which they refer to as Stalinists) were not actually following any sort of Communism at all.