Frames per Second

What is "FPS"?
Different than a computer's Desktop where an area is only redrawn when it changes (e.g. because you moved a window), the graphics of a 3D game are in most cases redrawn completely all the time.

This means that the computer clears the screen, then invests all his graphics and some processor power into drawing everything, and when it is done, the computer clears the screen again and draws everything anew. If the graphics are simple and the graphic card is good, this happens more than a hundred times per second. This is the Frames per Second (short "FPS") value that people make such a fuzz about. If the graphics are more complex and/or the graphics card is older and/or the computer is busy with something else, the FPS can drop down even below one.

How high should the FPS be?
For ok gameplay in calm situations, 10 to 20 frames are sufficient. In action situations, you'd definitely want something above 20. But so that you can really enjoy perfectly smooth graphics (think horizontal movement, for example), you'd want FPS above 90.

It makes sense to use the FPS limiter feature that comes with many games / game clients, because it is of no use to have the computer calculate 400 frames per second. The life of all involved components is less short if unnecessarily high FPS are prevented.

Buffering and vertical sync
The graphics are not drawn directly into the memory area that you can currently see on screen. Instead, they are drawn into a second memory buffer. And when the drawing is done, the graphic card switches from the currently visible buffer to the one with the freshly drawn graphics. The next time the graphics are drawn, the first buffer (which is currently invisible) is used. This principle is called "double buffering".

Games usually feature an option to have the buffer switching take place in the vertical blank. That is the moment when the raster ray of a CathodeRayTube Monitor is at the top, outside of the screen. Switching the buffers will then be perfectly smooth.

If the buffers are switched while the display device is somewhere in the middle of its refresh process (for example, when the CRT raster day is in the middle of the screen), the graphics look teared because both pictures can be seen, half of one at the top of the screen, half of the other at the bottom.

Turning vertical sync on, however, is not always a good idea because it causes the computer to wait a little while until the right moment for switching has come. This waiting is done via interrupts, so the computer is not really "waiting", but still this means that it takes longer until you see an updated picture of your current game situation on screen. If your FPS is high enough, though, you should definitely turn vertical sync on because the absence of the visual tearing feels much better.