Most people know that the United States Constitution and it's amendments guaranty "due process" and equal protection" to all of it's citizens. However, very few people know what that actually means.
In a nutshell, the Framers of the Constitution recognized that every law is going to infringe on personal rights. The Due Process and Equal Protection Clauses were included in order to limit how much any given law could infringe on these personal rights.
As such, when reviewing a law under these clauses, the Court must compare the strength of the government's interest in enacting or enforcing the law with the importance of the right infringed upon; i.e., the more important the right infringed upon, the stronger the government's interest must be in enacting or enforcing the law. If the right infringed upon is not a "fundamental" right, the law will be upheld if it is "rationally related to a legitimate government purpose" a very loose standard. If the right infringed upon is a "fundamental" right (i.e., as is the right to marry), the law will be upheld only if it passes a very strict test: there must be a compelling state interest and the law must be narrowly written to address that interest.
With that background in mind, the Due Process Clause says that no "State shall deprive any person of life, liberty, or property, without due process of law." This clause protects against the government taking something away or diminishing the rights of an individual without a fair trial, hearing or other lawful procedure. And again, the government can only take away the right if it meets the applicable standard set forth above (i.e., "rational basis" or "strict scrutiny").
Likewise, the Equal Protection Clause says that no State shall "deny to any person within its jurisdiction the equal protection of the laws". This clause seeks to prevent laws that either target a "suspect" class (i.e., race, ethnicity, gender, etc.) or distinguish between two or more classifications of people.