Need-based aid is calculated from your family's finances. Merit aid is awarded for grades, scores, talent or other traits regardless of income. Most schools offer some of both, but the mix varies enormously.
Where merit money is
Merit aid is largest at schools trying to attract students who could get in somewhere more selective. If your profile is above a school's middle range, expect an offer. The most selective colleges give almost none; they meet need instead.
Where need money is
Well-endowed private colleges and a growing number of flagship publics promise to meet full need, sometimes without loans. Each profile's scholarships section lists any such guarantee.
Stacking
Ask whether outside scholarships reduce grants or reduce loans first. The answer changes what an outside award is worth to you.