Supply-side economics is a kind of 'fad economics' grounded in the belief (disproved by the ineffectualness of the policy) that decreasing tax rates can increase tax revenues on the assumption that if people could keep a higher fraction of their income, people would work harder to earn more income.
Supply-side economics is a debate that can now be declared effectively put to bed as fanciful since it has been proved that the effective revenue generating effects of specific tax cuts would be at best 'relatively small' and certainly not enough to cover the difference in tax loses.
Supply-side economics is a theory grounded in the laffer curve, which states that lowering tax rates generates more government because it generates more economic activity, leading to increased opportunities for tax revenues.
Supply-side economics is, in fact, not about raising revenue (as it claims) but rather is actually intended to be a smokescreen for starving the government of revenues in the hope that the tax cuts would lead to a commensurate drop in government spending.
Supply-side economics is based on say's law, which states that supply creates its own demand. That is, it suggests that a product is no sooner created, than it, from that instant, affords a market for other products to the full extent of its own value.
Supply-side economics is different from classical liberal economics (despite its respect for hume, swift, smith, hamilton). Rather than arguing that taxation is the immoral seizure of one's property, as they do, it argues instead that the main impetus for tax cuts is an alleged collective benefit (i.e. Jobs).
Supply-side economics is a wrongheaded theory of 'free lunches' and is responsible for budget deficits during the regan administration, thus proving the laffer curve wrong.
Supply-side economics is ineffective in that the wealth of the super-rich does not trickle down to improve the economy, but tends to be amassed and sheltered in tax havens with a negative effect on the tax bases of the home economy.