To persuade is the first main type of author's purpose. Text written to persuade means the author's goal is to convince the reader to agree with the author. This might mean the author wants the reader to think or even act in a specific way. This type of writing is extremely common. Any piece which pushes a certain opinion or asks for some sort of call for action is persuasive writing.
It is true that in these types of writing the author shares his opinion, but usually he also provides facts and examples. This information serves to support the author's opinion and further convince the reader to agree with him. Examples of persuasive writing include speeches, advertisements, commercials and newspaper editorials. Any forms of propaganda are examples of pieces written to persuade. Look at this picture and identify how the author wants you to think or to act.
Inform
The second main type of author's purpose is to inform. In this format, the author's goal is to enlighten the reader about real-world topics and provide facts on those topics. However, in contrast to pieces written to persuade, these facts are not used to support a specific opinion. The facts are presented in order to teach the reader. Examples of texts written to inform include textbooks, cookbooks, newspapers and encyclopedias.
All these forms are written in order to provide information to the reader. It is also important to note that many authors present their works as informational texts, while at the same time inserting their own opinions into the piece. As a reader, be sure to be ready for opinions masked as information. For instance, some newspapers claim to only report the facts, but on careful inspection it is clear it might be trying to persuade you to believe a specific idea. Be on the lookout for these hidden opinions.
Entertain
The final main category of author's purpose is to entertain. This contains almost all examples of what you think of when you hear the word 'literature.' Authors who write to entertain have the goal of telling a story or describing real or imaginary characters, places and events. This includes all pieces of fiction, which is literature that is not real.