word choice - "which" vs "that"

Possible Duplicates:
Correct usage of which/that
When is it appropriate to use 'that' as opposed to 'which'?

I'm not a native English speaker and I must confess I'm quite confused about when I should use which in place of that and viceversa.

Is there a particular rule?

For example, let's take this sentence.

The method returns a Whois::Answer instance which essentially looks and behaves like a String but it's way more powerful than a string.

Is it which correct or should I have used that?

A few more examples:

It also includes a huge set of data parsers which converts a raw registry response into a pure Ruby object.

and

The only exception to this rule is the Timeout::Error exception which is intentionally left unmanaged.