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 wound onto spools.
The type of chemicals in the mixture can control qualities of the line. Things like limpness, strength, toughness and other desirable casting factors can be emphasized. The color of the line is also controlled by adding chemicals. Lines can be tailor made to fit a wide range of needs.
Limpness - A soft flexible line is "limp" and cast easily.
Strength - Line is rated in pound test and that is the amount of pull in pounds it takes to break the line. Most line will break at above the stated test. If you are trying to catch a line class record fish you must buy line that breaks at the stated test, called
Toughness - Monafilament line will abrade when rubbed against things like rocks and wood. Tougher line will not abrade as easily.
Stretch - Monafilament line will stretch, some more than others. Stretch can be both good or bad, so you need to choose the amount of stretch that is best for your fishing needs.
Diameter - The diameter of line is in fractions of an inch or millimeters. In general, thinner line is better since it is more limp, but it is not as strong or as tough as thicker diameter line.
Memory - Monafilament line tends to hold the shape of its spool, and this is called memory. Less memory in line is better.
Knot Strength - Monafilament line is slippery and many kinds of knots will slip and come loose. It will also cut itself, depending on the kind of knot. A good knot is very important and some lines have better knot strength than others.
Color - Clear lines are less likely to spook fish but harder for the fisherman to see. Some lines have a blue tinge in ultraviolet light so they show up above the water in sunlight but disappear under the water to offer the best of both worlds. Some lines come in bright colors so they are more visible and are good when fishing for species that don't shy away from line, like catfish.
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.