The Xperia ZL is the 'little' brother to Sony's recently-unveiled Xperia Z, and to get the most pressing question out of the way immediately: what's the difference between the two? Well, the ZL isn't waterproof (and thus has a different chassis and design), has a hardware camera button, a microSD card slot, and a a minutely larger (by 40mAh) battery. That's really it. The display, the chipset, the camera, and the software are all near as makes no difference identical to those on the Z.
Why make the ZL, then? Well, Sony has talked up things like different design direction and regional marketing strategies, but the reason this phone came to be is really quite simple: the ZL is slightly cheaper than the Z.