Your child at this age wants to try out everything and is likely to get frustrated and angry when what they expect does not happen. They might resort to aggressive behaviour. When they are playing they might cause physical harm to their friends or playmates, or throw tantrums to express their feelings.
What you need to know
Teach them to handle their feelings and handle the aggressiveness and frustration. Keep a watch when they are playing -- intervene before they get physical and divert their attention tactically. Teach them tricks to control their behaviour like counting to ten when they feel like hitting others, putting their hands in their pockets, getting themselves out of the vicinity. Teach them to control themselves by giving them time-outs (leaving your child for a certain amount of time to themselves after they have done something unacceptable).
Check if your child is angry, frustrated, hungry, tired, sick or jealous. These feelings might also lead them to be aggressive.