The simplest answer to your question is to explain that all pastel sticks are pure pigment mixed with binders in various degrees.
Hard pastels are as it says on the tin ‘hard’, this is because there is a greater amount of binder in them. They are very good for sketching and detailed work. Many artists will begin their paintings using hard pastels and move on to softer pastels as the painting progresses. Sometimes returning to the hard pastels for hard line detail and edges. And then use very soft pastels for highlights.
Soft pastels have less and less binder to varying degrees from soft to very soft.
You need to try out several manufactures pastels to arrive at sticks that you feel comfortable with or that suit your style or technique. Beware that you may buy a set of ‘Soft’ pastels expecting them all to be of equal softness, this is not always the case as some colours are harder or softer than others..
Hard pastels e.g. Conte, Faber-Castel Polychromos,
Soft pastels e.g. Winsor & Newton, Rembrandts etc.
Very soft pastels e.g. Shmick and Sennelier.
Hope this helps a little.