Spotlight is the name of Apple's built-in, system-wide search service. Originally codenamed Matador, was first announced for the Mac alongside OS X Tiger in 2004. It came to the iPhone as part of iOS 3 (iPhone OS 3) in 2009. It was the year after the introduction of the App Store but even in that short amount of time the number of apps people had on their phones had grown to the point where finding them wasn't always easy. So, in addition to the Home screen icons as app launcher they'd introduced with the first iPhone, they added a new screen to the left, accessible with a swipe, that you could use to type in app names and launch them that way. Spotlight could also be used to search for contacts, songs, email messages, and calendar events.
In iOS 7, Spotlight moved from a separate Home screen to every Home screen. Instead of swiping sideways to go to it, you swiped down to bring it to you. With it, you could search for apps, contacts, music, notes, events, mail, reminders, messages, and options to push the search out to the Web and Wikipedia.
With iOS 8, Apple is bringing context and location awareness to Spotlight.