Here is a comprehensive summary of the Wikipedia article on San Diego:

San Diego is a major coastal city in Southern California, located directly next to the Mexican border, with a population of over 1.4 million, making it the eighth-most populous city in the U.S. and the second-largest in California. Known as the "Birthplace of California," the area was originally home to the Kumeyaay people, and was first claimed for Spain by Juan Rodríguez Cabrillo in 1542, later named by Sebastián Vizcaíno in 1602 after Saint Didacus. The first permanent European settlement in California was established there in 1769 with the founding of the Presidio and Mission San Diego de Alcalá, before the region became part of Mexico in 1821 and was ceded to the U.S. in 1848. Today, its economy is driven by military and defense, tourism, and biotechnology, and it hosts major universities like UC San Diego and San Diego State University. The city anchors the San Diego–Tijuana transborder metropolitan area of roughly five million people, and is home to the San Ysidro Port of Entry, the busiest land border crossing in the world outside Asia, as well as the busiest single-runway airport in the United States.