Tier 1 schools are the top 25 schools of any particular category. Tier 2 schools are the next 25, and Tier 3 the next 25, and Tier 4 is the next 25. Everything else doesn't matter.
Harvard, Yale, Duke, Notre Dame, Georgetown, and Berkeley are all Tier 1 Schools.
MIT, Cal-Tech, Stanford, and Cornell are all Tier 1 Engineering schools.