“When, at the age of twelve, at the time of the Russian revolution, I first heard the Communist principle that Man must exist for the sake of the State, I perceived that this was the essential issue, that this principle was evil, and that it could lead to nothing but evil, regardless of any methods, details, decrees, policies, promises and pious platitudes. This was the reason for my opposition to Communism then—and it is my reason now. I am still a little astonished, at times, that too many adult Americans do not understand the nature of the fight against Communism as clearly as I understood it at the age of twelve: they continue to believe that only Communist methods are evil, while Communist ideals are noble. All the victories of Communism since the year 1917 are due to that particular belief among the men who are still free.” — AYN RAND, Foreword to We The Living
Communism is a social (political-legal-economic) system based on the altruist-collectivist principle that the individual has no right to their own life, but has value only in regard to their selfless (altruistic) service to the group, commune, or collective.
Politically, communism is a radical form of statism that is totalitarian/authoritarian (for examples of the totalitarian nature of communism see The Black Book of Communism.) Whatever “freedom” an individual has under communism is not by inalienable right, but by revocable permission.
Legally, communism is a pure system of regulation and operates under a “rule of man“, i.e., central planners, dictators, and bureaucrats. Far from being a classless society, individuals under communism are divided into two groups: those who give orders (master) and those who obey (serf).
Like all forms of socialism, communism leads to a destructive form of competition where individuals do not compete economically to produce values for trade, but compete politically to expropriate the values produced by others.
Economically, under communism, since one does not have a right to one’s life, one does not have a right the results of ones productive efforts: property. Thus, under communism, all property is owned, controlled, and distributed by state central planners under the principle of “from each according to his ability, and to each according to his needs.” Economically, communism is pure socialism in principle — and practice.
As there is no private property under communism, there is no basis for free-trade between individuals, and thus there is no market price system (except in the communist country’s “black markets.”) Without a rational price system (all prices are set arbitrarily by central planners) there is no means of rationally allocating capital (no profit-loss system to judge ones efforts) leading to massive capital malinvestment. This is the problem of the inability of “economic calculation” under all economic forms of socialism (See Ludwig Von Mises, Economic Calculation in the Socialist Commonwealth.)
As for the results of communism, quoting from The Black Book of Communism:
“[T]error has always been one of the basic ingredients of modern Communism. Let us abandon once and for all the idea that the execution of hostages by firing squads, the slaughter of rebellious workers, and the forced starvation of the peasantry were only short-term “accidents” peculiar to a specific country or era….”
“…[W]e have delimited crimes against civilians as the essence of the phenomenon of terror. These crimes tend to fit a recognizable pattern even if the practices vary to some extent by regime. The pattern includes execution by various means, such as firing squads, hanging, drowning, battering, and, in certain cases, gassing, poisoning, or “car accidents”; destruction of the population by starvation, through man-made famine, the withholding of food, or both; deportation, through which death can occur in transit (either through physical exhaustion or through confinement in an enclosed space), at one’s place of residence, or through forced labor (exhaustion, illness, hunger, cold). Periods described as times of “civil war” are more complex—it is not always easy to distinguish between events caused by fighting between rulers and rebels and events that can properly be described only as a massacre of the civilian population.”
“Nonetheless, we have to start somewhere. The following rough approximation, based on unofficial estimates, gives some sense of the scale and gravity of these crimes:
U.S.S.R.: 20 million deaths
China: 65 million deaths
Vietnam: 1 million deaths
North Korea: 2 million deaths
Cambodia: 2 million deaths
Eastern Europe: 1 million deaths
Latin America: 150,000 deaths
Africa: 1.7 million deaths
Afghanistan: 1.5 million deaths
The international Communist movement and Communist parties not in power: about 10,000 deaths.“
“The total approaches 100 million people killed.” 
 Martin Malia, “Foreword: The Uses of Atrocity,” The Black Book of Communism (1997).