Democrats generally favor the working class, the poor, the minorities, and are not afraid of taxing the wealthy and corporations. Democrats are very pro-Union.
Economically Democrats generally follow the Keynesian theory of economics. Which means they try to affect the demand side, feel that a little inflation is good, deficits are bad, tax cuts to the poor and middle-class, that due to the POT (Potential Of Thrift) that giving the wealthy money would not make them spend it or create new jobs, but instead save it. They believe in full employment and feel that unemployment is underrated. Keynesian economics helped get the country out of the depression, and created social programs. Democrats want a bigger government with more control.

Republcans generally favor the corporations, the wealthy (top 10%), they are afraid of taxing the wealthy and corporations. Republicans are anti-Union.
Economically Republicans generally follow the Classical theory of economics. Which means they try to affect the supply side, feel that inflation is bad, deficits are good, tax cuts to the wealthy and corporations, that saving is good and savings can be invested to create growth and fix unemployment. They feel that unemployment is overrated. Classical (or Supply Side aka Reaganomics) economics helped improve the economy in the 1980's when Ronald Reagan was in office. Some claim that part of Reaganomics are not true Classical economics, but Reagan did what he could to get congress to agree to his ideas. Republicans want a smaller government, and less government control, by removing regulations and let the economy or markets fix the problems. More of a hands off or lazie faire policy.

Notice I said generally, some politicians have a mix of ideas from both sides, or differ from these definitions in a nutshell.

Some Americans are in the middle like they could become a Moderate Centralist party member, but the Centralist party never quite took off, last I heard it was a Yahoo Group, and no offical web site was created. [link|http://groups.yahoo.com/group/centralistpartyofamerica/|http://groups.yahoo....stpartyofamerica/]