They both mean marriage.  You can think of them this way.

- 결혼 = marriage - focusing on the "union" between the two people.

- 혼사 = marriage seen as a passage or event.  사 = a matter or event: 경사(celebratory event), 일상사(daily matter), 대사(major event), 소사(minor event).

- 결혼식, 웨딩 = the wedding ceremony (식 = ceremony).

So 결혼 is used widely, including the verb 결혼하다, while 혼사 is only used in context of an event, preparation, expenses and the like.  혼사 is also a somewhat old and formal sounding term most often used by the parents of the people marrying (it implies a "family business" as marriage has traditionally been a family affair).  The couple themselves don't use this term very much.

Yes, you're exactly right.  혼사 is 결혼 행사.

It's great that you figured it out quickly :-) 

Thank you very much for the answer Tony.

I heard the term 혼사 for the first time today while watching a Korean drama and I did not understand why they used it instead of 결혼. The context was precisely the parents of the bride and the groom arguing about wedding preparation and whether or not the wedding should be called off.

Now that you explained it, the word 행사 came to my mind. 결혼 + 행사 = 혼사