After my recent debacle with the wrong needles, I figured there might be other numpties like me out there so a tutorial about choosing the right type of needle is in order. I've actually gone out and researched this one.
You basically have 3 big types of needles:
1. Needles for woven fabrics
These needles are sharp and can pierce fabrics. Singer lists these as type "2020".
2. Needles for knit fabrics
These needles are also called "ballpoint" needles and are designed to split the strands of fabric. Singer lists these as type "2045".
3. Needles for leather
These needles are sturdy in order to pierce leather pieces. Singer lists these as type "2032".
Remember that the little needle carton will say what the needle is for!
And you have several types of widths, from very narrow to very thick. The thicknesses, from narrow to thick, are:
1. 70/90 - these are used for very lightweight fabrics (organza)
2. 80/11 - for lightweight fabrics (voile, lawn)
3. 90/14 - these are used for normal weight (normal cotton)
4. 100/16 - these are used for heavy weight fabrics (coating, heavy wools)
There are some types of wovens which need even sharper than usual needles. Think denim. My denim needles came in size 100/16. Singer denim needles are type "2026"
Twin needles are used for pintucks and decorative stitches. The width between the two needles varies. Make sure you don't use a zig zag with them, they will break. Singer twin needle types are type 2025
Here's a very thorough link about Singer needles. It has more detail than my basic introduction.