Actually there is a progression.
- Gur
- Guru
- Sat Guru
- Siri Guru
Gur is the formula. It is the key. It is the Naam
Guru is the one who gives you the key. Guru could be a corrupt person. This person doesn't have to be perfect. He/she is just the person who gives you the key
Sat Guru means the words from this person will always be true on all 3 levels; personal truth, situational truth, and universal truth. You understand truth with every utterance of the Sat Guru
Siri Guru - There is only one Siri Guru. It is the Siri Guru Granth Sahib. The difference is that Sat Guru may come and go. Sat Guru may decide to isolate himself in meditation for a time. Siri Guru is always there, unchanging. Even if you insult the Siri Guru it will still tell you absolute truth. Siri Guru is above time and space.