This is what you know for a working certificate:
- You are dealing with the site that you see
- Your browser recognises both the certificate and the certificate authority
The transaction of your information is encrypted. However, the certifiate only guarantees identity of the site. It does not guarantee the reputation of the site (a popular belief).
In summary: if you can be sure of the site that you deal with, i.e., it has good past history and good reputation, etc., then you can be sure that the transaction is relatively safe because you know you are dealing with the site and only the site.
However ... keep in mind that most CC are not stolen through secure browsing, but through things like keyloggers. So you really have a lot more than just the browsing mechanisms to worry about!