Differential Signalling uses a pair of complementary signals to cary data, that is each wire is carrying the opposite signal, those signals are subtracted at the receiving end and the result is either 0 for noise or 1 for a valid signal. This technique is used with high speed signals to reduce crosstalk and EMI, examples of differential signalling include the twisted pairs inside Ethernet cables, HDMI and USB.
When routing differential signals, each pair should have the exact same length, because the signals are subtracted from each other so they need to arrive at the exact same time, otherwise it's useless, the same requirement applies to other high-speed non-deferential signals such as the signals of this DRAM, note the wiggly tracks on the PCB which are used to make the tracks of equal length:
Eagle 6 has some facilities to help with differential routing, if you use an older version you could still do it manually. For Eagle 6, first give both signals the same name with one ending with _N and the other with _P, so for example USB_N and USB_P, now when you start routing either one of the signals they both will be routed together as you can see:
If you can't route both signals together, if there's not enough space, then one will be shorter than the other, and that's when you will need the wiggly traces, or formally the Meander function. After you finish routing the signals as usual, run the meander function with the length you wish to add to the shorter trace, then click on the trace and move the mouse it will start adding wiggles to the trace:
You can also use the length.ulp to find out the difference between the tracks before using the meander function:
run length USB*