It's a differential pair. I'm surprised at the strange answers and arguments regarding twisted pairs and LVDS and things of that sort.
Differential pairs are notated symbolically as twisted pairs because, at a macroscopic level, that's how they're carried (think Ethernet, HDMI, USB, Firewire -- all use twisted-pair cables). And visually, it describes the signals as interconnected, which they are (they reference each other, not ground, after all!)
At the board level, differential pairs carrying high-speed digital or analog signals -- depending on their operating frequency -- are often routed as transmission lines with controlled impedances (both with respect to ground and with respect to each other). Regardless of operating frequency, differential pairs carrying digital signals should be routed with equal length, to prevent the positive and negative signals from arriving at different times.
Many low-speed analog signals are carried differentially as well; the most common example being audio, which is often carried over XLR cables, which use a twisted pair. For these signals, this transmission scheme is often called a balanced pair (or some variation on that). The balanced receiver uses the difference between the signals -- not their actual values -- to carry information. Because of this, noise rejection is extremely high, since any noise affecting one of the signals will affect the other in the same way (and will eventually be subtracted out altogether by the receiver).
In these low-speed signals, the matched lengths and transmission-line routing is less critical. Most importantly, keep the signals very close together, and always route them identically, so that any interference that affects one will also affect the other.
In the above image, the red traces are each differential pairs.
To interface with differential pairs, a line driver is generally used, which is a circuit that converts single-ended signals into differential, and vice-versa. One such circuit designed for digital differential signals is the SP3485 (an example in use would be on this SparkFun breakout board), though there are many others like it.