I'm not too sure why I didn't know anything about the Hoover Dam. (I obviously did not pay any attention in my history class.) With that said, I didn't know what to expect during this tour. Thank goodness there was a movie at the beginning of the tour to fill me in on the basics.
I had purchased tickets for the Dam Tour. After the movie, at our designated tour time, we met up with others on the same tour and those on the Powerplant tour. After a quick tour through the powerplant we met up with our Dam tour guide proceeded with the rest of our tour. (The Powerplant tour is shorter and cost $15, while the Dam Tour is an hour long and costs $30.) I think it's definitely worth the extra $15 to join the Dam Tour. We got to walk the tunnels in the dam, see instruments and markings only seen by people who've been on the tour and employees. Our tour guide was informative and funny -- he REALLY loves his "dam" jokes.
The tour takes you down in elevation so be careful if you have vertigo. Also, those who are claustrophobic I highly suggest you don't join any of the two tours. They really packed us in the elevators tight.
Note: Food and souvenirs are available for purchase by the parking structure. If you can, try to go early. Slots for the tours fill up fast and once the tour buses and RVs start rolling in, traffic can build up fast.