Charging by Friction

Charging by friction is a process when the electrons move from an object to another object. This process happens when you rub two objects with different materials together. Since the two objects have different materials, the atoms hold their electrons with different strengths and abilities

For example, you rub a glass rod with a silk. At first, the two objects are neutral and they have the same amount of electrons and protons. Since the glass rod loose their electrons easily, the electrons move to the silk and the silk becomes negative charged. The silk now contain more electrons than the glass rod.

Another example is when you rub your hair with a rubber balloon. The electrons in the hair moved to the rubber balloon so then, the hair is attached with the balloon because two of the objects have different charges.

How is electrical charge transferred by friction?

Actually, electrical charge is not created by rubbing two objects. Its only electrons which are transferred from one object to another.  

Resources:
http://www.physicsclassroom.com/class/estatics/u8l2a.cfm
GCE 'O' Level, Physics Matters, Marshall Cavendish Education