Must be some good stuff over at /r/anthropology

The difference is not one of time, but of intent and methods.

Archaeological excavations:

• Gain consent from property owners
• Gain consent from individuals or groups culturally connected to the site whenever possible
• Always seek to optimize the balance of knowledge gained versus dirt moved; we know archaeology is inherently destructive and we try to learn as much as we can while disturbing as little
• Dig to uncover information that is shared with other researchers, synthesized, and published or exhibited comprehensibly to the public
• Consider every little thing important
• Photograph, draw, record, map, document, and inventory as many things as possible, down to the color and texture of the dirt

While grave robbers (aka looters):

• Enter sites and dig illegally or w/o permission
• Show less concern for what their actions destroy
• Target only objects of value
• Work with only their own benefit in mind
• Don't bother to record what they're doing

There's no time limit on archaeology: if you excavate a 30-year old grave, it's still technically archaeology. However, you can learn so much more about the person/culture/time period in other ways. Why study someone's teeth to learn what they ate when you can ask their kid or look at their receipts? The biggest difference between archaeology, ethnography, and history is not one of time but of methods. Historians focus on written accounts, but those don't always exist. Ethnographers focus on observation and personal accounts, but that doesn't work for the past. Archaeologists focus on material remains, but those can only tell so many things. These three fields inform each other and are only hurt by solid delineations. Individuals will specialize in one or the other, but an archaeologist that never reads or talks with historians is only hurting his research.