Bullying can take many forms, but essentially it is a form of harassment such as the continuing intimidation or belittling of an individual either verbally and/or physically.
The TUC have discovered, though surveys that they have conducted, that those responsible for bullying at work include fellow workers and not just management.
Bullying may be wider, in that it may not necessarily be open, or specifically aimed at particular individuals. For example, a domineering, excessively critical style of management is a form of bullying, which could lead to stress in other workers.