Taxes and Economic Policy
This is perhaps one of the most contentious issue areas. Basic disagreements arise from different philosophical approaches to what is considered a desirable amount of governmental intervention into economic matters. Put most simply, liberals tend to see government as a force for good for the collective society, and see taxes as a reasonable means of getting everyone to pitch in for programs that are in the interest of the public good. Conservatives fundamentally disagree, and would prefer far fewer governmental services, and the lower taxes that go along with that. Thus, we have much debate over these issues.
Regressive vs. Progressive Taxes
Progressive taxes: Taxes can be levied in a way that has people at the bottom of the income scale pay less as a proportion of their income than people at the top. This is called "progressive taxation", meaning that as you go up the income scale, people pay progressively more. Since people with lower incomes use a larger proportion of their income on basic needs, and have less discretionary income, this is considered most fair to progressives/ liberals, and some moderates.
Regressive taxes: Regressive taxes put less burden on those with higher incomes and more on those with lower incomes. One justification for such a tax structure is the idea that lower income people use more government services, and therefore ought to pay more. This type of tax system is more likely to be favored by conservatives.
Interventionist vs. Laissez Faire Capitalism
Conservatives tend to want less government oversight and intervention on economic matters, whereas liberals/progressives tend to want more regulations and rules. Where that line is drawn depends on the individual, though most Americans would like to see some regulations (anti-monopoly rules, rules about working conditions, etc), though certainly something short of full government control of economic matters, which very, very few Americans would condone.
Read up on Tax and Economic Policy
How does American fiscal policy compare to other countires? This article in Foreicn Affairs by Political scientist Andrea Louise Campbell puts it in perspective.