Can Socialism and Communism can be used interchangeably?

Some people, especially people who don't really understand Communism or Socialism, do often use them interchangeably (or, curiously, as completely unrelated terms), but it's technically incorrect. Communism is just one branch of Socialism; there are many branchs. It's much like how Catholicism is but one branch of Christianity, or Trigonometry is one branch of Mathematics.

What is tricky is that until about 100-ish years ago, Communists usually called themselves Socialists. The reason being that their belief that Capitalism needed to turn into Socialism, and then it would eventually turn into Communism. So they were trying to get their society to take that first step and become Socialist. Around the end of World War I, the Communists finally separated themselves from mainstream Socialism, although they still often used Socialist terminology (as seen in the Union of Soviet Socialist Republics, or USSR).