What's the difference between lead score, urgency, and priority?

These are controlled in Admin > Sales Insight.

Urgency=Flames=percentile of score acceleration (lots of points recently?)
Priority=Stars=percentile of Score across database, compared to all other leads - is this score in the top 2.5%?

You can modify these if desired.

Someone please correct me if I'm wrong: 

Score is just that, the score (sometimes depending on your own scoring methods this can be demographic + behavioral scoring into 1 single score). Set by your own operational programs within Marketo.

Relative score is a measure from 1-3 on how this score compares to all other scores in your instance. Populates as STARS in Sales Insight (may depend on how your specific settings are setup though, as this can be changed)

Urgency: a system score that is based on HOW RECENT the user is active with your content....that is, how recently is this scoring happening? Intended to give someone who downloaded 3 whitepapers TODAY a higher score than someone who downloaded 3 whitepapers last week.

Relative Urgency: again, a 1-3 measure on how this urgency compares to others. Shows up as FLAMES in Sales Insight. 

Priority....hmmm...this one I am not entirely sure about.

They are all related in the way Naor also described it.

Thank you for clarifying. What are the max values for urgency and priority?