What is an open tuning?
An open tuning is called 'open' because when you strum all of the open strings, they make a chord together. Open D tuning, for example, makes a D major chord (DADF#AD). Open G tuning is tuned to a G chord (DGDGBD). Open E is like open D but one step higher. Drop tunings are not open tunings, they're basically half open and half standard.
When does one use an open tuning?
Technically you can use open tunings any time you want to, and I encourage you to because it will take you out of your comfort zone and force you to try new things in new ways.
On average however, open tunings are used a lot in slide guitar playing. Open tunings are very friendly to the across-the-neck nature that a slide necessitates. Anywhere you place the slide along the guitar neck, you can play a chord with one finger, and simply move one or two frets above or below to play notes in between the chord tones.
Who uses open tunings?
In order to learn the nuances of different tunings, I recommend two things: Experimenting for yourself and also studying the masters. Here are a few famous slide guitar players and what they used. Learn their licks and use it in your own playing.
Duane Allman, Sonny Landreth & Derek Trucks- Open E
Elmore James- Open D
Keith Richards- Open G
Johnny Winter- Open G, D, and A