Semicolon separates two complete sentences, but they should be closely related. It can also separate two longer sentences that are linked with a longer coordinator SV; therefore/however, SV. They must be complete sentences. She talked; I listened. Compare: She did the shopping, and I did the cooking. She did not feel good; therefore, she stayed home. What comes after a colon is usually a list. What comes before must be a complete sentence. She had many many items on her grocery list: eggs, milk, chicken, cheese, and yoghourt. The dash, like the comma or parentheses, is used to insert a thought into the middle of a sentence. It should not be used often.