Strictly referring to the United States political arena, the Democratic Party is left of center and the Republican Party is right of center. How is this measured? It is somewhat arbitrary in that it relies on the history of the country to determine what left and right really mean; but if you picture an arc with the far left being Communism and the far right being Libertarianism, the middle is a 50/50 mix of the two. An example of how the left/right bar is arbitrarily set is that the Democratic Party in the United States is more right wing than the New Democratic Party in Canada. Another way to understand the difference is to think of democrats as liberal and republicans as conservative. Please understand that these are broad terms that don’t necessarily reflect the specific policies of any one politician.
Here is the nitty gritty. The Democratic Party in the United States seeks to provide a national health care plan and to offer more government supported social programs to help the working man. They tend to be more tolerant of different religions and are less focussed on building the military. In general, they support a higher income tax in favor of universal health care, welfare and retirement protection. The Democratic Party is the oldest and largest party in the United States and was founded by James Madison and Thomas Jefferson. Environmentalism has become an important focus for the Democrats.
The Republican Party was founded in 1854 and the first Republican president was Abraham Lincoln. The Republican Party in the United States supports a free market economy with less government intervention. They favor lower taxes and less social programs. The Republican Party in the United States is divided between fiscal republicans and social republicans. Fiscal Republicans favor less government intervention in personal finances and free enterprise. They believe that we have a right to decide what to do with our money and shouldn’t be tightly regulated. Most importantly, they believe in individual liberty with regards to life and personal property. Social Republicans in the United States believe more in old fashioned family values; religion features prominently in their lives. They are pro life and anti gay marriage.