The final phase is the destructive phase. In this phase the virus can mess up your screen or delete your entire file system. When that file executes, the virus copies itself onto other files and then later once some time passes, inflict the damage. Others infect special tracks on a floppy or hard disk called the system or boot sector. Every time you do a directory they get activate and copy themselves.
Some viruses are actually hybrids of both types.
There are many thousands of viruses and each one has a different set of tricks up its sleeves. Some actually change themselves to evade detection. Yes, as a file attachment. You have to run the file before it can do anything. No, you can't get an e-mail that you read that causes a virus in your machine.