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.
Monsoon Lagoon Waterpark. A storm is brewing over the shores of Lake Erie and Sandusky Bay, and no one, no one single person can avoid getting wet. Are you ready for splashtastic fun? It's Monsoon Lagoon Water Park with over three-hundred thousand square feet of pure, wet excitement.