Similarly you would speak about trees or bugs or frogs differently with your high school biology teacher than you do with your younger brother. Finally, you often need to make presentations, introductions, or arguments differently in front of different audiences. For example, arguments about public policy would be made differently to Republicans than to Democrats, an introduction to ethics would presented differently depending upon whether your audience was composed mostly of fundamental Christians or skeptical agnostics, and nuclear waste management issues would be explained differently if the audience were land management experts or attendees at a town meeting. There is also a reason that the speech is happening, the occasion for which the audience has gathered. Each of these occasions has different norms for speaking, calling for speakers to operate in different modes from formal to informal, from light to heavy, humorous to serious, conversational to highly practiced.
Speakers hope to accomplish general and specific purposes when they communicate.
The line between informing and persuading is not absolute, and many speeches will do some of both. When you seek to inform , you want your audience to walk away from your speech knowing more than they knew beforehand. You may want to explain an idea or process to them, to give them new information, or to show them how to do something. Certainly your introduction of a fellow student will be largely an informational presentation. When you aim to persuade an audience, on the other hand, you want them to adopt a new position or belief, to change their minds, to sway their opinions, or to be moved to action.