Some would call this bungee cord. Its nothing but elastic and simple cordage found at Wal Mart. I've tested its strength over many trails, in clothing, pack covers, and tarp set ups.
Extremely light yet strong, it can aid in setting up various configurations of tarps. It also comes in handy when setting up on platforms where wild camping is not allowed. Vermont is such a place, on the Appalachian Trail.
I always cut to length I need, and heat seal the ends with a match or lighter.
The crocheted cordage, last example was created for extreem conditions, and strapping sleeping pads to an external frame pack.