I'd like to give you a couple of real world examples, better than a picture.
Orthographic projections are used to represent a 3 dimensional object in 2 dimensional views.
Grab a CD case and set down on the desk. The view that you see when you look straight down at it is called the front view. Now, if you where to pick it up from the left hand edge and rotate it 90 degrees in a clockwise direction, this view is the side view. A drawing of these two views would have the front view in the center and to create the side view, a line is projected from the top and bottom edges to the left. These are called orthographic projection lines and in this case set the top and bottom boundries for the left side view. All projected views are drawing in 2 dimensions, A width axis running right to left called the X axis and a height axis perpendicular to the X axis, from top to bottom called the Y axis.
An Isometric view is defined by all three axis being rotated equally.
The Z axis, which is perpendicular to both the X and Y axis. It is only visable in the side view as the depth of the CD case. Now, take the CD case by the top edge and tip it up towards you till it's about 1" about the table keeping the front edge on the table. Then, using the bottom right hand corner as the center point, rotate in a counter clockwise direction until the lower right hand corner has moved about 1" up. You now can see the front view, side view and depth in 1 view.