Guideline(s) vs rule(s)

A guideline is just that - a guide. It indicates the general direction.

http://www.dictionary.com/browse/guideline?s=t

any guide or indication of a future course of action:

Rules, on the other hand, attempt to constrain behaviour by setting limits and parameters.

http://www.dictionary.com/browse/rule

a principle or regulation governing conduct, action, procedure, arrangement, etc.

I would say that they can be used synonymously, but it's not entirely correct. "Rule" is much stronger than "guideline", as the latter is an encouragement as to what should be done versus the former as a hard indication of what should be done.

Rule translates to a straight measure in many languages derived from Latin. Something that is rigid and determines the same measurement or requirement to everyone. Unwritten/unspoken rules cause problems to everyone not apprised thereof.

Guideline has two meanings depending on one's background. In general usage, it refers to a guide line or a rope that leads the (right) way, whereas others maintain it is a rope that raises a tent straight, thereby being a vital structural component which cannot be ignored. The second meaning is used if you have ever been camping (successfully) or in the military or been reprimanded by someone who has. The context needs to be established at each conversation.