Title VII of the 1964 Civil Rights Act made it illegal for employers to treat workers differently based on race, color, religion, sex, or national origin. The ADA added disability in 1990; the ADEA covers age 40+; the Genetic Information Nondiscrimination Act (GINA) added genetic data in 2008; and the Supreme Court's Bostock v. Clayton County (2020) confirmed that "sex" under Title VII includes sexual orientation and gender identity.
The protection covers everything that touches the job: hiring, firing, pay, promotions, assignments, training, layoffs, benefits — pretty much any decision your employer makes about you. You can't be legally fired, passed over, paid less, or harassed because of who you are, no matter what reason your boss puts on paper.
The hard part is rarely the law. It's the proof. Employers almost always offer a non-discriminatory reason ("performance," "restructuring," "not a culture fit"). Discrimination cases get won by showing the stated reason doesn't hold up — through timing, comparator evidence, shifting explanations, or patterns across the workforce.