Why are Republicans (or those who favor capitalism) called the right" or "right-wing" and Democrats (or those who favor social issues) called the "left?""

After the French Revolution of 1789, the French National Assembly became the governing body and liaison between the king and the French citizens. When the National Assembly met, members of the Third Estate — who were the revolutionaries and those who wanted change, and were thus "liberal" — adopted the practice of sitting on the left side of the Speaker; while the members of the First Estate — the nobles, who were more inclined to want to keep things as they were, thereby "conservative" — sat on the right.
Some say the practice originated from an old custom in which honored guests like noblemen would be placed to the right of the host at formal dinner parties. Either way, the practice of liberals being on the left (of a chamber, philosophy, and so on) and conservatives being on the right has carried through to other parliamentary bodies, including the United States Congress, as well as our political lingo.