It's important to note there are 4 main types of public speaking. These are speech purposes. Anything can be used as a speech topic. You must first consider the general purpose you want your topic to fit into before you set out writing your speech.
The four main types of public speaking, or speech purposes, are: The entertaining speech, the informative speech, the persuasive speech, and the inspirational speech.
The entertaining speech usually comes in the form of a story of some sorts. Many people confuse an entertaining speech as a presentation only to be used for stand-up routines and the like. Although that is one example of how an entertaining speech purpose could be used, it is not the only way. Entertaining speeches are stories full of word pictures and other strategies to evoke sensory recall. Stories which bring you right into the moment as if you are actually there.
The informative speech is a presentation that's mainly technical in nature. In this sort of speech you would be delivering facts, statistics, algorithms, etc. An instructor teaching a course on rebuilding engines or explaining the inner workings of your computer would be giving an informative style speech.
The persuasive speech is giving presentations in arenas where you would like people to take some kind of action. Buy a certain product, vote a certain way, donate money to a certain cause. These speeches are employed by salespeople of all kinds, politicians, etc.
The inspirational speech is a presentation which calls on the audience to be the best that they can be. These speeches are made at high school graduations, inaugural addresses, etc.
A few things to keep in mind concerning the types of public speaking. When developing your speech, choose a general speech purpose for your topic and make it crystal clear both at the opening as well as the conclusion of your speech. Otherwise your audience will leave frustrated or confused as to what it is you expect of them. It's important that the cohesion at both ends binds the entire presentation together.
In the body of the speech it is equally important to incorporate as many of the other types of public speaking as you can. By doing this you add life to your entire presentation, bringing the audience with you on a journey. Sticking to only one type through the entire speech will leave it much too inauthentic and boring. An inspirational speech is more powerful when sprinkled with a few facts and some light humor, just as an informative speech would be much easier to listen to with entertaining anecdotes embedded in the presentation