Merriam-Webster defines an inference as drawing a conclusion or opinion that is formed because of known facts or evidence. An inference is a statement or belief that a person concludes about something unknown but is based on something known. For instance if a person were to smell smoke, she could assume that there is a fire some place or something that is causing the smoke, without actually seeing what was causing it. This is an educated guess based on the information that the person has in that moment.
The ability to make inferences is innately human. People draw conclusions based on certain information even without having been given all of the information. Inferences can purposely be used in writing or may be something that a writer chooses to avoid. When writing on a research topic, for instance, the writer may be more inclined to stick with substantial evidence and facts for supporting a claim, rather than giving limited information to help the reader draw inferences.
Understanding inferences can help writers develop strong essays--and particularly argumentative essays--or when the writer is trying to support a claim. Most people prefer to have some kind of proof when they are told a statement. It is up to the writer to be specific and to give his readers the information that they need to draw their own conclusions. Inferences are important for the writers and the readers.
Writers should be aware of what messages may come forth from their writing, whether or not those messages are intentional. A writer can either persuade or turn her readers away with the inferences that the reader might draw from the information. This is important when writing to a certain audience or when trying to convince the reader of some kind of opinion. Writers send messages not only through the written words, but also through the unwritten message that the reader can find within.