Pixels are the actual dots that create the text or images.  Within each pixel are the actual lights or diodes called LED's (Light Emitting Diodes). 

Pixel Matrix is the arrangement, or array, of the pixels (Ex: 16 x 64). 
16 pixels tall (or rows) by 64 pixels wide (or columns).

Pitch is the distance between pixels.  Some companies use the term °resolution±.
Most small LED displays have one LED per pixel and large LED displays can have anywhere from 4 to 25 LED's per pixel.

Pitch determines the size of the text. A 17mm pitch displays 5± text as compared to a 34mm pitch which displays 9± text.

As the distance between pixels increases (the pitch), the more LED's per pixel is typically required, so the text is fuller in appearance.
Typically, a wider pitch is required for LED Displays that need to be read from a long distance.

• 16mm is visible from under 100'
• 20mm is visible from 100' - 250' (depends on the size, could be even more distance)
• 25mm is visible from 250' and more

Viewing Angle, typically means, how far the user can walk from center facing the LED display in either direction and still see the same level of brightness .  Most companies refer to viewing angle as readability and not brightness. 

There are two common types of LED's on the market: a 30 degree diode and a 70 degree diode.  This degree refers to the brightness.

On a 70 degree diode, pass 35 degrees from center, the brightness will fade a bit, but the LED Display is still readable up to about 70 degrees from center, creating a 140 degree °readable viewing angle±.

On a 30 degree diode, pass 15 degrees from center, the brightness will fade a bit, but the LED Display is still readable up to about 45 degrees from center, creating a 90 degree °readable viewing angle±.

Solid State LEDS are 10 times more efficient than incandescent bulbs and about twice as efficient as fluorescent lamps

The LED display also has a much greater viewing distance, especially at night, which allows your message to reach more people for more hours of the day. Unlike changeable lettering signs, the LED display allows you to update your messages quickly, easily and safely, without concern about weather, and eliminating the ongoing cost of replacing standard lettering.