What guagua means is many things. Like I said in a post ages ago , they can be family pick up trucks. However, they are also a form of public transportation. It may still be a pickup truck but you will be paying (and you may be riding in the truck bed).
Now the type of guagua you take depends on the route and how much you are willing to pay.
For a short route that is less traveled in the rural areas, you will take a pick up most likely.
For a short route in the capital you will take a very old large capacity (from 3 rows of seats to around 8 or 10) van. It will be a caliente (hot), so there will be no air conditioning and when you get stuck in traffic misery will commence.
Between cities there will be both callientes and expresos (express) guaguas. This depends on the length of the route and the volume of people. To San Cristobal, I have the option of taking a caliente or an expreso. The difference in price is 15 pesos (less than 40 cents) but the difference in time in the trip can vary by 45 minutes. That is because an expreso generally will have a part of the route it does not stop during. The expreso also is generally in better condition and will have air conditioning. Yesterday I grabbed one and was surprised to find not only a movie (not the first time I have seen one on my bus, but still I don’t understand when the ride is generally under an hour), but also wi-fi. I tested it with my kindle and it worked. I talked to Ashley E. and she said about two weeks ago she first noticed it on her bus to Bani. Bani is the next major town west of San Cristobal. Taking a guagua from San Cristobal to Bani, there is no expreso option.
For an expreso to a major destination along the southeast coast you will actually get a ticket (sometimes with a seat number) and climb on a coach sized guagua. The one to Higuey shows a movie, one time I saw a dubbed Rocky.
The final option for longer travel is through Caribe Tours or Metrobus. These are the Peter Pan/Greyhound companies of the Dominican republic. For these, you have to get on at a stop and buy a ticket beforehand. Other guaguas unless they are passing through a union regulated part of the route where they cannot stop, will pick you up on the side of the road when you flag them down.
Final thing to know is that unless you are on a bus that sells tickets, the seats are put in for very thin humans, and not evreyone in the country is the size the seats are, so you may be touching someone for part of the ride or you may even be contorted to fit.