Both Communism and Socialism begin at the same point.  To make the distribution of wealth equal, government must take charge of wealth.

Thus, Socialism is defined as:

a theory or system of social organization that advocates the vesting of the ownership and control of the means of production and distribution, of capital, land, etc., in the community as a whole (from here)

And Communism has much the same definition as socialism.  In fact, Communists say the ultimate goal of communism is a socialist state.  To the extent there is a difference between the two, that difference comes from those governments that have called themselves Communist.  Their practices have given rise to definitions such as these.

In practice a self-identified Communist is a Socialist willing to bring about a socialist state using force.  Why are Communists so willing to use force?  Supposedly, the Communist seeks paradise on earth. Karl Marx phrased it this way.

From each according to his ability, to each according to his need (or needs). (from here)

Communist also advocate atheism.  Although atheists claim atheists can be as ethical as those who believe in God, the history of communist societies does not support such a contention. Communists have shown little willingness to let anything get in the way of their idea of paradise on earth. Unfortunately, no Communist government has ever set up a society that approaches paradise on earth.

Socialism has achieved a bit more success.  Why?  Instead of attempting to achieve Marx’s paradise on earth using force, Socialists have settled for implementing their economic goals incrementally. Thus, in the United States, for example, government’s role in providing (either by operating or via heavy-handed regulation) charity (welfare), educational services, medical care, home mortgages, public utilities and so forth has steadily expanded.  Inevitably, however, Socialists also encounter the same basic problem that foiled the Communists.  Putting so much power in the hand of government officials corrupts them.  Ronald Reagan, The Great Communicator, explained the dilemma in a single sentence.

Any government powerful enough to give you everything you want, is powerful enough to take from you everything you have. — Ronald Reagan

Socialism and Communism share the same basic flaw. Neither the Socialist nor the Communist can answer the following question.  What gives government the ethical right to redistribute the wealth? If government has no such right, then we are guilty of using the ends to justify the means.  That has always been recipe for trouble.

When put into practice, both ideologies entitle government officials to override the individual’s right acquire and own property.  Without the right to own property, we cannot own ourselves.  When we become slaves of the state, we cannot pursue our own idea of happiness.

–Tom Salmon