When you head down to the sporting goods store or bait shop, the choices in fishing lines can be staggering. But by knowing where you're fishing and what kind of gear you'll be using can quickly narrow those choices. There are three major categories of fishing line and different situations for using all three.
Monofilament, or simply "mono" as it's sometimes called, is created by a single nylon strand. It wasn't that long ago that mono was the only option available to fishermen, unless you wanted a super-heavy-duty braided line - usually reserved for deep-sea fishing.
Mono is the most affordable option and is often available as a clear, nearly invisible color, making it a good option for fishing stillwater. It does tend to get frayed and weakened fairly easily, though so fishing it in a rock stream could become frustrating, since you will have to often re-tie your lure and cut off damaged line to prevent the line breaking on a snag or large fish. And since I recommend using ultra light tackle you are looking at only using four to six pound test line, so you will really need to stay on tops of this damage.
The flip side of that is that it is the most affordable, so if you are short on cash, you can certainly get by using it. And if you are fishing especially clear water, or a still water pond, it may still be your best option.
2. Braided Line
The second major category of fishing lines are braided lines. These once were just used by saltwater fishermen looking for super-strong lines to catch huge fish, but soon manufacturers such as Berkley and Spiderwire began marketing these to freshwater fishermen as a super-strong option. You will often see these advertised as "15 lb. test, 6 lb. diameter" to show the line's strength relative to monofilament.
There are downsides to using braided line however. Braided lines tend to be super-slippery, so you need to use the knot recommended by the manufacturer. Many fisherman even add a dab of superglue onto the knot to ensure it doesn't come untied when you hook a fish. Also, braided materials don't have the same stretch as mono, so while it takes a bit more for the line to get damaged (due to it's super-strength) if it does, the line's strength is immediately negated and you are actually more likely to have a break once the line is frayed than you do with damaged mono.
Braided lines tend to take a bit more care than standard monofilament as well. They get weakened if they are twisted or get a kink in them - both are kryptonite to braided line's super-strength. Braided line should be inspected every time you reel in or release line (if you are not casting) to insure you don't have any problems like these.
This kind of line is a legitimate choice for trout fishing in streams, but you need to be aware of the line's drawbacks, so you aren't caught by surprise losing a lure on a small fish or having your line break quickly on a snag.
3. Fused Lines
Fused lines are a "middle ground" between braided and monofilament, often advertising the strength of braided line with the ease of use of mono. These lines are often sold in the monofilament category but are made of several strands of polymer fused together to make an extra-strong version of standard monofilament(or an easy-to-use version of braided lines).
Berkley Fireline (shown) and Spiderwire both have fused versions of their braided lines, but be sure to read the box carefully as both also have standard braided and monofilament versions. For Berkley look for the "fused" label and for Spiderwire check for their "Super Mono" label (their EZ Mono is a standard monofilament - and has not gotten good reviews).
Fused lines are also a middle ground in that they are less expensive than braided lines, but more expensive than monofilament. For most trout fishermen, though they make for an excellent choice for fishing heavy cover, such as you'd find in a typical trout stream.