How do I make sure that duplicate data isn't stored in a table?

In other words, if record 1 has the email address "bob@aol.com", I want to make sure that no other records that are created can have the email address bob@aol.com.

Right now, I'm solving this problem through PHP by checking if that email address exists before inserting it. But isn't there a way to do this through mysql? I though there was a certain way to set up a table so that duplicate data couldn't be inserted.