Braided line was one of the earliest types of fishing line, and in its modern incarnations it is preferred for some situations because of its high knot strength, lack of stretchiness, and great overall power in relation to its diameter.
Braids were originally made from natural fibers such as cotton and linen, but natural fiber braids (with the very rare exception of braided silk) have long since been replaced by braided or woven fibers of a man-made materials into a strand of line. Braided fishing lines also have good resistance to abrasion, which is very important when fishing on rocky terrain.
Braided lines also have 1/3 to 1/4 the diameter of mono or fluorocarbon lines at a given test breaking strength. Therefore, it is easy to fit much longer braided line on a spool than monofilament or fluorocarbon line for the same strength. This is very important for deep sea fishing, since reels don’t have to be very big to accommodate long lines. Also, thin braided lines provide less resistance to sea currents.
The newer synthetic braided lines, can be used on any type of fishing reel, but are excellent for bait casting reels, particular when trolling.
Sea Fishing How-To has a nice guide to lines at Types of Fishing Lines – What fishing line should I use?
Bonus: How to tie the strongest knot for braided lines.