Im going to write a few of these, this one is not finished but ill post it anyway.
here's a link to many many many more.
This guide is being written to answer the questions that come up all the time, and require a lengthy post to answer. I will attempt to provide pictures when possible, examples of guns, pros and cons of different styles and systems.
I will add a mechanical markers section later; here is the e-gun section.
Electronic guns are just that, electronically operated in one of three ways.
1. Sear tripping solenoid. Most commonly used in guns not designed to be purely electronic. An electric solenoid applies force in the same manner as you pulling the trigger would. It either pushes back, or pulls down on the sear that releases a hammer and opens some form of poppet valve.
A spyder:
ALL spyders use this system, as well as all variations like the pirhana. The downsides to this system are a combination of things. Not only is there generally more kick with these guns, there are more parts to break, springs that wear out, a hammer/sear assembly to wear out, limitations on cycle speed of the solenoid required, and power consumption from that solenoid.
2. Electro pneumatic/blowback hybrid. These guns, such as the wrath or BKO, use a small air solenoid to drive a ram forward with air from a low pressure regulator to open the valve. A spring then returns the ram/hammer to the "cocked" position.
3. Fully electro pneumatic. These guns are designed from the ground up to operate only with an electronic solenoid providing air to a ram. There are several different version of this system, but they all operate off the same principle. A solenoid valve allows air into a piston (or ram) assembly and drives the bolt forward. When the bolt enters "battery" the valve (be it poppet or spool) fires a ball and the solenoid reverses the flow of air to the ram, driving it to the rear. That design is standard for all higher end guns. IT gives greater efficiency, lower kick, and potential for higher speeds.
A matrix(spool valve style system)

A Intimidator(poppit style)
There are some variations from those basic setups.
An Autococker for example, utilizes not only a sear/spring return hammer combination, but also utilizes pneumatics to recock itself. That comes from its own unique history and evolution; it started life as a pump:
Another hybrid is the E-mag, and its home-brew relatives. It is one of the few sear-trippers (sear tripping solenoid) that does not release a hammer, but releases the bolt and pushes the on off assembly:
There are also other variations like the MQ valve, which I have neither the experience, nor technical understanding to adequately explain to you how it works.
The board
All these designs are actuated ("set off" so to say) via the same basic method.
When the trigger of an "E"-gun is pulled, a micro switch, either optical, magnetic, or mechanical, is actuated that tells the gun to fire. This switch transmits a signal to a controller board the same way a mouse’s button, or a keyboard would. That board then activates the guns firing cycle.
Several things determine what goes on there that determine whether the gun fires and how many shot happen per-trigger pull.
The Dwell setting on a board determines how long the solenoid stays activated (i.e., how long it stays in the "on position").
The "firing mode" determines how many shots a gun is capable of, how many will come out per trigger pull etc. There are "ramping" modes that will begin to add shots as you sustain a certain trigger pull speed. There are also "full" auto modes, as well as bursts, and a multitude of other "extra" modes, some of which are legal in certain tournaments and fields, and require special consideration of the rules.
De-bounce is a feature common to many boards that helps keep extremely sensitive triggers from bouncing and tripping the switch by itself while the gun is firing. This is necessary to stop the gun from firing uncontrolably, as bounce in a trigger is illegal in most tournaments.
"Eyes" on and off. As far as I’m concerned, eyes are the greatest thing to come out of the "E" gun generation. These are optical sensors in the breech of the gun that detect whether a paintball is in the breach. With them equipped and turned on, the gun will not fire unless a ball is in the breech ready to fire. It’s allowed unholy rates of fire to be possible without balls being broken by the action of the bolt.
There are many variations on "E" guns, and this is by no means perfect. Ask for clarifications if need be, I definitely do not mind answering.
Ok, I'm looking for pictures of several things. Any animations of internals, pictures of LCD displays, the internals of a spyder frame, noids etc.
Like the other thread, Please PM me corrections, or additions, I will credit you when there added, i jsut dont want to spread the information out over the whole thread. Im perfectly willing to accept cirtiscism, but lets keep it civil

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Like before, all material is original and my intelectual property, please ask me or give me credit if you reproduce it.
The animations I've added are from Deadlywind.com's tech section. If there's a problem with me remote linking them(on DW's part) lemme now.
Thanks to snow for the new cocker animation
Cory Watkins, proudest member of the fightin Texas Aggie Class of 2008