As the name suggests, the guitarlele is a cross between a classical guitar and a ukulele. It is tuned like a guitar capoed at the fifth fret (ADGCEA). This gives the top four strings the same tuning as a low G ukulele. The guitarlele sounds closer to a classical guitar than it does a ukulele.
There aren't a great number of guitarleles around. The most common is the Yamaha GL-1 Guitalele. The only other guitarlele that I'm familiar with is RISA's solid guitarlele (you can see WS64 demonstrating one here). More recently have brought out a guitarlele which they call a u-tar.
They've had a bit of exposure in the UK recently as they've been used by Jake Penate and The Maccabees (you can watch The Maccabees' Orlando Weeks playing First Love on his Guitarlele here).
Wilfried Welti plays Triumphmarsch on his Yamaha guitarlele.