The texture of your hair can be defined into 3 categories: Coarse, Normal (or Medium), and Fine.
By definition, the texture of the hair is circumference, or width of a single hair strand.
Coarse hair texture has the largest diameter of all textures and contains a medulla (one of the 3 layers to the hair shaft). Most male facial air is coarse, for example. It is the most difficult texture to lighten and will not damage easily. Heat temperatures for styling tools should be between 380 - 410 degrees F. It is the strongest of all hair textures.
Normal, or Medium hair textures are the most versatile. It may contain a medulla if the pigment of the hair is natural dark. Styling tools should be set between 360 - 390 degrees F. This hair texture is typically the easiest to style.
Fine hair texture has the smallest diameter and is the most fragile. Caution should be taken when coloring as it lightens quickly when bleached. Styling tools should be between 340 - 370 degrees F. This hair texture can be damaged easily from rough brushing and tight hair ties. It also tends to lack volume and body.
On the other hand, hair density means the number of hair follicles on any given square inch of scalp. This could be defined as thin, medium, or thick hair density.
You can see a chart of this on the "What is the Density of My Hair?" Blog.
It is common for stylists to get the texture and density confused, but when it comes to hair extensions - it's very important to understand as this will give you the best, most natural results when apply hair extensions.
Here is a great chart to help determine which texture & density box you fall into: