Thread is one of those things that, unless we're choosing it for the top quilting, we don't think about much. We have "our thread" and we like what we like and that's what we use. Me? I tend to stay with Guterman 50wt. black in the top thread, and that, or Superior Bottom Line gray in the bobbin. I vary very little from that, but I'm starting to find that yeah, I really do need to change it up sometimes depending on what I'm doing. But honestly, I didn't know how I should be changing my threads and why, so I did a little bit of research about the different threads, when and why they're used. I hope you find this as helpful as I did!
Thread Basics 101
Yes, just like wire, thread comes in "gauges," referred to as the thread's weight. The higher the number, the finer the thread. A good middle weight thread, and most commonly available is 50wt. and it will get the job done with most moderate thickness fabrics, like flannel, cotton, batik. 60wt. is more intended for bobbin thread, or sewing with lightweight fabrics like chiffon. A 40wt. thread is best served for things like cordouroy, canvas, and denim; its thickness will prevent it from breaking as it is pushed through the heavier fabrics.
Cotton really is your go-to thread. It works well for most projects, is quite durable and comes in every color under the sun and is reasonably priced. Cotton thread is "Mercerized" meaning it has undergone a process to give it smoothness and a bit of luster and it accepts dye well. It is always wise to use thread to match your fabric, since with lower quality threads (as with fabric) the dye can bleed. It is good to use when you're sewing cottons, rayons or linens. Cotton has very little give to it, so if you're sewing knits or anything in which you need to retain stretch in the seams, cotton is not the right thread for the job.
Polyester is widely available, and the subject of hot thread debate (who knew?). There are some who swear by polyester and who will use nothing but, and there are others who completely refuse to use the stuff. I suppose it comes down to personal preference, as with most things, but consider your purpose. Polyester doesn't shrink or fade, because it is a synthetic fiber, and as such it is intended for use with other synthetic fibers. It works well with knits, polyester, and cotton.
It is lustrous and a great thread for decorative stitching. It is a synthetic fiber with little give, so it's not suited to anything with stretch (unless in the capacity of embellishment, not seams). It does have a tendency to slip out of machine needles and break, so it can be a little trying in the patience department, but the effect is worth it. It can be used in seams in a pinch, but it's not really the best choice for the piecing work.
Nylon thread is meant for synthetics when used in a seam. It is mainly used for top quilting, as it is "invisible" and very forgiving as it embeds in the fabric well to hide stitching errors. While after completion it is very strong, it can snap when sewn in too fast. Some nylons are iron safe, and some are not - it depends greatly on quality and whether they are coated with another heat-safe substance or not. Use a Metallics needle for best results in a sewing machine.
Silk is perfect for hand applique. It hides well in the fabric and glides through the pieces like a dream. It is on the spendy side, though. It is best suited for "luxury" fabrics because of its cost and its natural sheen. It is also very strong, but you will also find a more limited array of color. It also works well for garments, because it can easily be hidden and it isn't bulky.
Wool thread is meant less for seams and more for embellsihment and hand applique. The common wool traits are applicable here - it is thicker, coarser and more expensive than your usualy cotton or polyester. You can use them in a machine, but you will need a larger eye for your needle and they fuzz like crazy, so clean out your bobbin case regularly.
These are meant for embellishment and top quilting only. It snaps easily, making it a beast to work with, so this is not at all suitable for seamwork - besisde, you won't see that gorgeous shine in a seam anyway. These have a very striking effect on a quilt or garment, but can over power too. To cut down on snapping, use a Metallic needle in your machine for best results.
As with all choices in material you can make, consider your purpose. Choose a thread that is best suited to what you're doing and how your item will be treated. Take into consideration the type of fabric you're sewing and its strength versus the strength of the thread - if they are too different in durability, the thread can wear through the fabric over time and create holes, or the wear on the fabric will snap the thread. Needle Pointers suggests that you choose a thread of the same matrial as your fabnric, and the same size as the weave of the fabric to ensure equality in strength. They also suggest, if equality isn't possible, to choose a thread weaker than the fabric - a broken thread is easier to repair than shredded fabric. Always test for colorfastness and be aware of how it will shrink in the wash versus your fabric. If it shrinks more, then it will create puckers which gives a quilt a more antique look. If it shrinks less, the puckers aren't created. Do you want your stitches to show or not? If you do, use a contrasting color or a thread with a different luster than your fabric; if not, choose the same color or luster as your fabric.
Remember, just like fabric, you get what you pay for. While 5 for $1 thread is a bargain for your wallet, it is as cheap as it is inexpensive, and unless it is a major brand on markdown, it will tend to snap, stretch, shrink, fray, fade, and ravel more than the higher quality (and higher cost) threads which will in turn affect the quality of your work. There are many high quality threads out there, and you have to try several to find what you like and what your machine likes best. I prefer Guetermann or Presencia for my top thread, and Superior Bottom Line for my bobbin thread for piecing and applique (we have these at the shop). I'm really partial to Superior Monofilament and Sulky Holoshimmer for decorative quilting. You'll have to find your thread brand of choice for yourself and change up your fiber content for the project at hand.
Sources and More Information
All Free Crafts' article on Thread
Textiles in Depth
Needle Pointers - explore this one...there is A LOT of information.
Best Sewing Machines' Thread Tips