To put it clearly, instructional design is the design, development and delivery of instructional materials (books, instructions, lectures, courses, educational systems, etc) with the goal of making the action of learning a certain subject more efficient. All of this practice is supported by learning and cognitive theories as well as behavioral psychology.
To understand it better, let's imagine the following situation. Suppose you have a private university and you want to make your students acquire skills and knowledge more efficiently. To do so, you'll look for an instructional designer. He will be responsible for applying a methodology with the goal of making the process of learning more efficient, of applying the instructional design.
To accomplish this goal, the instructional designer will apply a model of instructional design. These models are responsible for establishing a list of steps to follow, in order to assist the instructional designer in obtaining its goal. The most common one being the ADDIE model which consists of five phases:
- Analysis (determining who are the learners, the context, the delivery options, etc);
- Design (planning the lessons, the objectives, subjects, etc);
- Development (creation of the tools and content based on what was planned);
- Implementation (application of all the tools and material to the learners and possible teachers);
- Evaluation (this occurs during all the process and at the end of it).
At the present-day, instructional technologies (such as websites, online tools, computers, the internet, etc) are also being used to support learning.
I hope my little imaginary situation helped you in understanding the definition of instructional design!