Traveling across USA having entered using the visa waiver program, having been approved for an ESTA. If I enter Canada for a few days, and then re-enter the United States, does my 90 days refresh upon re-entry?
Generally, you do not get a new 90-day period on VWP if you have only been to Canada, Mexico, or adjacent Caribbean islands since your last stay on VWP. So you would only be admitted until the expiration of your previous 90-day period.
However, if you travel beyond those countries, e.g. it you go to Central America, and then return to the US, you can get a new 90-day period of admission on VWP, and there is no "limit" to the number of times you can enter on VWP in a 2-year period. (Of course, whether to admit you is still at the discretion of the officer, and if you visit the US too frequently you are more likely to be denied entry for immigrant intent.)
No, not only does it not 'refresh' it it, but the days spent in Canada (and Mexico) count to the 90 days.
See answer to us visa waiver program - Leaving Mexico after 90 days as part of ESTA requirements for more details.