In experiments, behaviour is the observed reaction which occurs when an organism is given a stimulus. Some organisms are more complicated than others and may be more aware of their surroundings. If their behaviour is conscious it means that they know what they are doing. If they are not aware of what they are doing this is unconscious behaviour.
Humans will pull their hand back without thinking if they accidentally touch something hot. This is done because we (and other animals) inherit nervous system reflexes which cause us to pull away from the thing that is causing the pain.
Behaviour is linked to the nervous system as well as the endocrine system. The nervous system reacts, and notices what happens. The hormone system is much slower. It makes some kinds of behaviour more, or less, likely. A simple example is the change from being a child to an adult. This involves a whole range of hormones which affect growth and behaviour.
Behaviour is also linked to learning. Organisms which are more complicated can learn better than those that are simpler. However, even the simplest animals show habituation, which is a primitive kind of learning. For organisms, learning means that they change their behaviour because they remember having been in that situation before and know how to react.
People often use the word "behaviour" to mean the way people interact with (behave towards) one another. Children are taught what is good behaviour and what is bad behaviour. Good behaviour is about being polite and thoughtful to other people. It is the kind of behaviour that people in a society expect from others.