CBT is really an umbrella term that covers most of the techniques used by professional psychologists. The basis of CBT is the understanding that our thoughts (our cognition) determine our feelings and our actions and that our thoughts, our emotions and our actions and behaviours all interact together. It is really a circular process, our thoughts impact the way we feel, which then determines our behaviour, the outcome of which influences our thoughts and so on.
When people seek psychological help, it is generally because they have negative or unrealistic thoughts that are causing them to feel in distress or out of control, creating actions and behaviours that lead to negative outcomes. These negative outcomes reinforce the negative thoughts and the cycle deepens.
CBT allows people to be aware of these negative thoughts and teaches strategies that help you develop alternative ways of thinking and responding that reduce the psychological impact of those feelings and behaviours. This helps break the cycle so that you can begin to feel in control of your thoughts, your feelings, your behaviours and choices and take control of your life again.