A complex sentence is a sentence consisting of an independent clause and a subordinating clause. Or, to put it more simply, complex sentences involve more than just a straightforward noun + verb. Complex sentences include extra phrases added to a base sentence, but which cannot stand alone as a sentence. For example:
Published 4:00 am, Thursday, January 9, 1997. 1997-01-09 04:00:00 PDT Yosemite National Park -- Yosemite Valley is slowly drying out after what park rangers are calling the worst flood here in recorded history, leaving behind hundreds of upturned tent-cabins and acres of wrecked camping sites.
Yosemite Firefall. The Yosemite Firefall was a summer time event that began in 1872 and continued for almost a century, in which burning hot embers were spilled from the top of Glacier Point in Yosemite National Park to the valley 3,000 feet below. From a distance it appeared as a glowing waterfall.