Behavior is defined as the actions and mannerisms of individuals or systems in response to internal or external stimuli. Behavior modification aims to alter behavioral patterns through techniques like positive and negative reinforcement. Positive reinforcement strengthens behaviors by rewarding them, while negative reinforcement strengthens behaviors by removing unpleasant stimuli. Punishment weakens behaviors by introducing unpleasant consequences. Behaviorism theories like classical and operant conditioning explain how behaviors are learned and changed through their consequences.