Trim is a quick cheat to make an item look fancier. It can also hide funny hems and other issues. It is most often applied to the edges of garments such as collars, cuffs, skirt hems, etc., although sometimes they are used to create stripes or other patterns.
Trim varies immensely in price. Much of it is sold by the yard, and prices range from about $1.50 to $20 per yard. I average about $3/yard with my materials. There are lots of options on the low end of things, including all of the examples to the right.
You can also get some trims on spools. Be sure to check the length each spool holds. My local stores carry spools that are both 4 feet and 6 feet in length, which makes price comparison complicated. Spools generally hold fairly inexpensive materials.
Trim goes a long way. Even a very plain trim can add a lot to a piece, so don't go overboard (and that's about the only time you're going to hear me say that!). I've never used a trim more than 5/8", such as the wide black trim depicted here. The small gimp braid here is 3/8", and the other two are 1/2" in width.
Black is my default color because it goes with everything. If I worked in lighter colors, I would say the same about white. Browns, however, are a little more tricky, as there are lots of shades of brown, and they don't all match. Colors are trickier still, because the green braid shown here, for example, will still not match a great many green fabrics.
Bringing a sample of your fabric to the store can be helpful so you can hold the trim against it and see if they work well together.
Trim colors don't even necessarily match other trim colors. Consider the green trim, the heavy gold braid, and the yellow gimp trims here. The gimp is totally out of place. And I actually got the gold braid to match the gold and green trim. Most metallic golds are much more yellow, and they don't match these examples at all. Again, visually compare materials before purchasing them.
Patterns can very quickly go over-the-top. Be careful about buying them.
Then there's the case of metallics. Gold and silver should make things look the richest, right? Well...sometimes it just makes you look tacky. Use metallics in moderation. The project which used the above metallic trims was meant to be over-the-top. Otherwise, I never would have used them; I would have used the yellow gimp instead.