The most common actions to return are these:

When you return pass the request and subsequent response will be passed to and from the backend server. It won't be cached. pass can be returned from vcl_recv.
When you return hash from vcl_recv you tell Varnish to deliver content from cache even if the request othervise indicates that the request should be passed.

pipe .. XXX:What is pipe? benc

Pipe can be returned from vcl_recv as well. Pipe short circuits the client and the backend connections and Varnish will just sit there and shuffle bytes back and forth. Varnish will not look at the data being send back and forth - so your logs will be incomplete.
Deliver the object to the client. Usually returned from vcl_backend_response.
Restart processing of the request. You can restart the processing of the whole transaction. Changes to the req object are retained.
Retry the request against the backend. This can be returned from vcl_backend_response or vcl_backend_error if you don't like the response that the backend delivered.