Formal and Informal Tone

Instructional designers across the globe mainly use two styles (or tones) of writing while developing content. These styles are formal and informal. The formal style or tone will sound more sophisticated and indirect yet straightforward. The tone will be more suggestive than instructive. On the other hand, the informal tone will read more casual and direct. Thus, it's more of instructive than suggestive.
To better understand the difference between them, let's consider the following example taken from one of G-mail's feature:

"Never file another message or create another folder. Use Gmail's built-in Google search to quickly find the email you need."

Its formal version would be:

"Use Gmail's built-in Google search to easily locate your emails. It also relieves you from the tasks of filing new messages and creating different folders."

Some more useful tips for identifying the informal tone would include:

1. Use of contractions (you'll, you're and it's)
2. Beginning sentences with conjunctions (and, but, or)
3. Using phrases or incomplete sentences
4. Missing prepositions like commas at certain instances

The type of tone you should choose for your content solely depends upon factors such as subject matter and audience. If the subject matter is technical in nature or related to some kind of management, the formal tone is preferred to retain clarity of the concept and avoid misinterpretation of the facts. The audience for such subjects are also generally well-versed with technical terminology and does not need heavy explanations. In such cases, you want to provide complete control to the learner so the tone is always suggestive. However, informal tone is generally preferred for subject matters that are related to soft issues. Here, audience is generally non-technical and prefers guided form of learning. The tone is also suitable for audience whose first language is not English (or the language in which content is delivered).

Also, it's a mis-(perception) that the informal tone makes use of colloquial and slang.