Lisa Akbari on Hair Texture.

In her book, The Black Woman's Guide to Beautiful Hair, Lisa Akbari states the need to set aside discussing "grades" of hair and instead be attentive to "real information that will empower your hair."
This focus is the hair strand, which tells us the texture and type of hair. She notes that textures come in fine, medium and coarse. As texture relates to heat damage she writes:

"Fine strands cannot tolerate excessive pressing or heat from a curling iron, blow-dryer, flat iron, pressing comb or heat rollers. Fine hair will damage easily because it has fewer protective layers than medium and coarse hair strands. The smaller strands requires the least amount of maintenance products in order to manage.
Medium hair strands are of average size, have a medium quantity of layers and can tolerate heat and chemicals better than fine hair. Although this is true, all hair strands burn if the temperature of a heated tool is greater than 150 degrees. So you must be careful not to overuse heated styling tools or chemicals. I recommend that you avoid pressured heat, instead try a lower more indirect heat.
Coarse hair strands are the largest of the three are thicker and are difficult to damage...."

You can listen to Lisa's radio show on this site: http://www.blogtalkradio.com/Lisa-Akbari

Getting it straight: Coarse vs. Fine

Tara asks, " Why does my friends type 4 hair get straighter and stay straighter than my type 3 hair if we use the same heat setting?"

Hey Tara,
The outcome of your results actually have more to do with your hair texture, rather than the type.
You may be surprised to learn that your hair is likely more coarse than your friends. That is, the measure of the circumference of your hair strand is larger than hers. Coarse hair is stronger and less susceptible to damage. It tends to resist processing and straightening.
Conversely, fine hair is very easy to process and is most susceptible to damage from chemical, heat and daily styling. Medium or normal hair doesn't carry any special tendencies (so lucky).
One person can have a mixture of textures on separate areas of their head. You can also find the various textures throughout all hair types. However, a large percentage of type 4's commonly fall into the fine-normal category. Of course, we all know of some exceptions.

Hair Typing.

Having a basic knowledge of your hair type is a tremendous help when searching for products, inspiration, tools and when trying out techniques. Having a general sense of your hair's tendencies, needs and what it will and will not do will save you a lot of time and money in your hair journey. Only in my dreams is a shake and go possible. Honeyfig.com has a great article on hair typing to get you started.

There's also a misconception that I've shared along with many other natural women. It's the belief that thick hair (densely packed) means coarse strands. That is usually not the case. In fact, African Americans tend to have fine, fragile strands. So in order for you to fully describe your hair texture and type I'll further provide you with a little vocabulary from Urbanbella.com.