(Adobe Photoshop) What is the difference between air brush, paint brush, pencil tool ?

Open a blank page and experiment with them: select one, choose a colour, choose a size and see what they do. The airbrush simulates a paint spray (as from an aerosol), the paint brush simulates a paintbrush and the pencil acts as a pencil i.e. it will only draw lines (but you can alter the thickness of the lines). The other two are usually used to fill large areas.