The Concept of View Projection
Imagine you are creating a drawing of something that needs to be documented. In this case I'll use a car because most people can easily visualize one. Imagine that you need to create multiple views of that car so that someone else will able to build it.
You would need at least six views of that car to show all the parts in a measurable manner. You would need the Front View, the Right Side view, the Left Side view, the Top view, the Rear view and the Bottom view. Six views at minimum to show the vehicle accurately.
Imagine that there is a box around the car and the front, sides, top, bottom and rear views of the car are printed onto the inside of the box. When you unfold the box to reveal the interior you have the basic drawing layout used in drafting. There are possibly additional views, but these six are the fundamental elements for a design drawing.
America's Standard - Third Angle Projection
Third Angle Projection is the one of the means that multiple views are projected onto the interior planes of the imaginary six sided cube. This system is often described as visualizing the part on the inside of a bowl. Sitting at rest at the bottom of the bowl and looking straight down into it you see the Top view and as the part is slid up one side or another it reveals other views in turn.
Third Angle Projection drawings have a symbol showing the type of projection used. All drawings should have this symbol.
Personally I prefer this type of visualization, mostly because I'm used to it.
First Angle Projection Symbol
First Angle Projection
You tend to find First Angle Projection layouts in Europe. There is a symbol that should be on every drawing showing what system was used with the word METRIC prominently displayed.
This system uses the same six sided cube and bowl concept, but with a twist. And this is where it gets you.
The bowl is upside down.
That means that the image in the view is as if you are looking through the part not at the part. You are seeing the far side of the part on the plane, you are not seeing the side of the part facing you.
This takes a lot of getting used to and it is very easy to screw up the part when fabricating it. I had a report that my drawings, which were intended to be fabricated in the USA, were instead sent to Germany where they produced mirror imaged parts because the Germans failed to note the projection symbol. Oops.