How do you represent three dimensional object in two dimensional screen ? Orthographic view is the answer. Perspective view is when you look at the object and object looks like its really three dimensional on your screen. Imagine that you are standing on the middle of the train track. when you look at the track ,it seems like its coming from the single point which is your perspective view but in reality its parallel lines which is orthogonal view.
Parallax and its consequence
To understand orthographic view and perspective view, some terminologies such as parallax needs to be understood. Suppose you are measuring the length of a line segment using your length measuring scale. when you look to the length from the front , it might show exact 4.8 cm ; but when you look exactly from the top making 90 degrees, it might show data around 4.8 cm e.g:4.9 cm or 4.71 cm but not exact 10 cm.this is the example of parallax error. Parallax is displacement or difference in the apparent position of the object due to different line of sight. Here is the wikipedia link if you want to consolidate your knowledge on parallax.
Orthographic view is view without parallax error. It represents three dimensional structure in terms of two dimensional screen. Its not a real world view , rather its a view that is good for engineering applications , creating three dimensional models where accuracy is demand, because there will be no parallax error. simply saying orthographic view doesn’t have perspective distorsion. While using blender, you can use orthographic view to model objects; for example : modelling a house , creating a phone case to 3d print etc.
Perspective view is view with parallax. The real world view that your eyes are currently seeing is perspective based view. It depends upon the line of sight of viewer. While in blender Perspective view can be seen during rendering and animation. The sole purpose of perspective view is to give you idea about what it would look like in real life.
How to switch between Perspective view and Orthogonal view in Blender?
To switch between perspective view and orthogonal view press 5 on your numpad or go to view->View Persp/Ortho