Image from craftzine.com
The variety of thread sizes, fibers, and styles is truly compounded by the sheer number of colors available!
I sometimes catch myself just staring at a thread display gazing from shade to shade!
In the beginning of my quilting adventure, I kept to cotton thread only for piecing and quilting, using the exact same thread in the bobbin as well.
(yes, I was a cotton-only quilt snob)
There's nothing wrong with this approach! Simple is good as well as cost-effective! There are just so many varieties and once you give yourself permission to try new threads, you will see which ones you like and how the right thread can really enhance your quilts.
Thread sizes are simply this: the larger the number, the finer the thread. Example: a 50 wt.(weight) cotton thread is larger in diameter than a 100 wt. silk thread.
I use Mettler 50/2 cotton (purple labelling) for piecing. I use both the cotton and silk for quilting, but my favorite thread to quilt with is a 40 wt. polyester thread (Isacord and PolyX are the one I have) generally sold for embroidery. I love the subtle sheen and the way it can disappear into a quilt or stand out depending on the color. In my bobbin, I use the same 50 wt cotton in my bobbin with the polyester and use a 60 wt cotton with the silk.
I am always on the look-out for new threads, new tools, like new fabric, can be a starting point of inspiration!
So, what are the differences?
Well, to me the cotton thread stands out on the quilt top because it is larger. The polyester can blend or shine depending on color, and the silk can become almost invisible since it is so fine. I recommend that use by a spool of each and try them out to determine what you like. I keep all three on hand so I can change the look of my quilting to suit the design of my quilt. I have used all three on a sampler to keep as a tool for matching my thread to my project.
Be sure to use a new needle in the right size for your thread. Thread sizes correspond to needle sizes. Use a small needle with a fine thread, I use my 60/8 needles with the 100 wt silk, a 75 Microtex or 80/12 with both the 40 wt polyester and the 50wt cotton.
REMEMBER TO CHANGE YOUR NEEDLE OFTEN!
All 3 of these, cotton, polyester and silk, are colorfast in my experience. I haven't tried Rayon, but it is beautiful! I really am trying to keep a check on my toolbase. If you use rayon, let me know how it works for you.
Some terms you may see with threads are "mercerized" and "trilobal".
A mercerized cotton thread has been treated to enhance it luster.
The beautiful, rich, lustrous batik fabrics are dyed on mercerized cotton base cloth.
A trilobal polyester thread has 3 rounded sides which will catch the light, also enhancing luster. Variegated thread (like Sulky Blendables) has multiple colors spaced on each spool.
These are not RULES. I offer guidelines, a place to start. Be adventurous and try that beautiful lustrous thread you bought because of the color!
More in depth thread info is available here and here.