When to Use a Dash

A dash is a mark of separation. It is stronger than a comma, less formal than a colon, and more relaxed than a parenthesis. 

When he looked at the drink-that was bubbling and churning-he wondered if he should drink it at all.

The parrot made a noise-a wheezing, choking, exasperated cough. 

The bald spot that caught the sun, the hunch in his shoulders, the limp in his walk-all the signs of old age catching up with him. 

Only use a dash when a more common mark of punctuation seems inadequate.