The goal of speaking effectively results in the intended message being heard and understood and then acted on. There are two main elements to speaking effectively:
What you say and how you say it.
What you say means your choice of words. When talking one-on-one to a friend, the words you might use will likely differ from those used in a formal presentation or interview.
How you say the words might vary with the usual way that you speak in different situations. Your normal tone and volume of your voice and your body language will likely change based on your audience or your own feelings about what you say.
Effective speaking means being able to say what you want to say so that it is heard and acted upon. To accomplish this requires considering every possible tool and aspect to ensure that nothing distracts or detracts from your message.
Whether you are talking to a major conference about a new scientific discovery, your children about their behavior, or your boss about a pay rise, you need to be able to speak effectively.