1) Strict Scrutiny:
Strict scrutiny is a form of judicial review that courts use to determine the constitutionality of certain laws. To pass strict scrutiny, the legislature must have passed the law to further a "compelling governmental interest," and must have narrowly tailored the law to achieve that interest. A famous quip asserts that strict scrutiny is "strict in name, but fatal in practice."
For a court to apply strict scrutiny, the legislature must either have significantly abridged a fundamental right with the law's enactment or have passed a law that involves a suspect classification. Suspect classifications have come to include race, national origin, religion, alienage, and poverty.
Source:
http://www.law.cornell.edu/wex/strict_scrutiny
2) Intermediate Scrutiny:
Intermediate scrutiny is a test used in some contexts to determine a law's constitutionality. To pass intermediate scrutiny, the challenged law must further an important government interest by means that are substantially related to that interest. As the name implies, intermediate scrutiny is less rigorous than strict scrutiny, but more rigorous than rational basis review. Intermediate scrutiny is used in equal protection challenges to gender classifications, as well as in some First Amendment cases.
Source:
http://www.law.cornell.edu/wex/intermediate_scrutiny
3) Rational Basis:
Rational basis review is a test used in some contexts to determine a law's constitutionality. To pass rational basis review, the challenged law must be rationally related to a legitimate government interest. Rational basis is the most lenient form of judicial review, as both strict scrutiny and intermediate scrutiny are considered more stringent. Rational basis review is generally used when in cases where no fundamental rights or suspect classifications are at issue.
Source:
http://www.law.cornell.edu/wex/rational_basis