Monofilament fishing line has been around for years and are the most popular kinds of lines. they work for a wide variety of fishing applications and can be made to have specific qualities that help under varying conditions.

A mixture of polymers are heated until fluid and then extruded through tiny holes to form strands of line. The size of the hole controls the diameter of the line as well, to some extent, the pound test of the line. These strands are cooled quickly and would onto spools.

The type of chemicals in the mixture can control qualities of the line like limpness, strength, toughness and other desirable casting factors.  The color of the line is also controlled by added chemicals.  Lines can be tailor made to fit a wide range of needs.

Lines come in a wide variety of qualities. The more expensive lines have better quality control and will be consistent in strength, color and diameter.  They will hold up longer on the spool and in use.  

Monofilament line stretches, which can be bad or good.  Stretch makes line more forgiving when a big fish makes a strong run, but it also makes it harder to set the hook.  The amount of stretch can be controlled by the additives but all monofilament will stretch some.

On spinning reels a limp, flexible line is best but lines with those qualities are usually not as strong and stretch more. Stiffer line works well on bait casting reels and it can be stronger and have less stretch.  Stiffer lines are also usually more abrasion resistant. Stiffer lines hold the shape of the spool more and this memory causes problems casting at times.  It also makes the line coil after a cast and lowers your ability to feel and control your bait.

Monofilament is cheaper and works in a wide range of fishing needs.  It is very popular and will probably be your best choice for general fishing activities.