In general, the harder or more abrasive something is the more likely it is to cause damage or trauma.
When toothbrushing this trauma can happen on the gums (gingiva) or the tooth surface.
It shows up as:
Gingival abrasion-the gum is cut or scratched.
Gingival recession-the gum is pushed further along the root surface, so more root is showing.
Cervical lesions-the enamel/dentin has started to form an indentation. You may be able to feel it with your tongue or fingernail.
These lesions may become sensitive and may require more extensive treatment like a gum graft or a filling; it is not reversible.
Be Mindful Of Your Habits
When you are purchasing your next toothbrush be mindful of your habits and consider a soft or extra soft-bristled toothbrush to prevent unwanted damage on your oral tissues.
Canadian Dental Hygienists Association Position Paper on Tooth Brushing
Canadian Dental Association