Not much difference really.
The PRD (Partido Revolucionario Dominicano/Dominican Revolutionary Party)
was founded by Juan Bosch, in exile from the Trujillo dictatorship a long time ago. He came back to the DR in 61 right after the assasination of the dictator (I was there when he arrived in his hometown, La Vega, when I was 12--I had no idea who he was then). This man was a very honest intellectual who leaned to the left but was nowhere near the Communist that the U.S. government and the DR right tried to portray him as being. They successfully used it to throw him out of power 7 months after winning the first elections after the end of the dictatorship in 1962.
He started to lose control of his party as it grew and the undesirables started taking over and perverting the good ideals under which it was founded. No better proof of its steady decline than the lousy PRD government we had from 2000-2004; and the other 2 previous PRD governments. Total disaster.
In 1973, Juan Bosch founded the PLD (Partido de la Liberacion Dominicana/
Dominican Liberation Party) which was, until the last few years, a small clique of people educated by the party to serve the country. It has grown in size recently and is becoming tinted by some of the riff-raff types that abound in the other two parties. They have governed from 1996-2000 and are now in power, as you know, and, in my belief, the best far and above the other two major parties.
The third, PRSC (Partido Reformista Social Cristiano/Social Christian Reform Party) was founded by the dictator's own long-time collaborator who inherited his legacy and governed us for 22 of 30 years from 1966 to 1996.
This was Joaquin Balaguer who was born in 1906 and was governing us into his 90's, blind for many years and almost a total invalid (probably the only case in modern world history and shamed me then and shames me now that Dominicans could not find one other person to lead them!). Their alleged philosophy is a party of the people, just like the PRD, but basically it is a conglomeration of personal and party interests looking for ways to steal the people's resources.
There are many small parties that don't add up to anything but work their 'magic' to obtain favors from the 3 big parties as allies and share in the feast of the government pie. As simple as that.