Bullying is...

Throughout history there has been 3 main forms of bullying:

Physical

Non-Physical Verbal

Non-Verbal 

The above can be either direct or indirect.

Below in the table there is a classification of the forms of bullying which was found in Rigby, (1996, p. 20):

Although the above three forms are the main forms of bullying , there are also sub forms of bullying which include racist bullying, sexual bullying, bullying of special needs children, sexual preference (Sullivan, 2000) and the form that has become more common; cyber bullying (Trolley & Hanel, 2010). 

References:

        Rigby, K. (1996). bullying in schools and what to do about it. Melbourne, Victoria: The Australian Council for Educational Research Ltd.

        Sullivan, K. (2000). The Anti-Bullying Handbook. Auckland, New Zealand: Oxford University Press.

        Trolley, B, C. & Hanel, C. (2010). Cyber Kids, Cyber Bullying, Cyber Balance. United States of America: Corwin.

        [Photograph]. Retrieved July 28, 2015, from http://www.google.com/image