The Triarian Guard
What is the Triarian Guard?
Basically, one of the last functioning remnants of the great empire. The Triarian Vexil was the preferred ship of the Triarian Guard. These ships keep the peace and protect the space that was once known as being the main part of the empire.

Where are they, and why are they there?
Right now, if you are operating in the center star cluster, where you start as a new player, you are operating in the space governed by the Triarian Guard. This is meant to allow new players the chance to learn how to control their ships and begin learning the ropes, without being blown away by an older, possibly much more powerful player that they could never defend themselves against.
It is always very easy to know exactly what areas they control though, by going to your "Options" button, that is always at the bottom of your screen. Click on the "Space controlled by government" box, and you will automatically have on your screen all areas controlled by the guard shown with a green circle around that space, as shown below. In the center star system, every planet has a green circle around it, because they are ALL protected and controlled by the Triarian Guard.

What happens if I try to attack someone in the Guard's
space?
If you try aiming any weapons at another ship in the areas in green (the area controlled by the guard), you will be first be warned by the Triarian Guard that this is not allowed in their territory, by seeing this warning:

If you choose to continue... you will then have commenced an attack not only on the ship you are targeting, but on the Guard itself, since you are in their governing space.
They will not put up with this kind of hostility in their space, and will stop you and protect anyone in their area.
This means, if you foolishly decide to continue your
actions, you won't be able to attack the ship you targeted, and you will be
rendered "Harmless".
They won't go so far as to pod you... but they will disable parts of your ship
that you really need, like weapons and engines or the like. It will
definitely be costly and time consuming to repair your vessel, though you
they'll be gracious enough to still make it possible to, at least.
You'll also be followed by one the Guard's ships (which are the fastest and most powerful of ANY in the universe to date, and cannot be outrun), and you'll be forced to endure them keeping an eye on your vessel for a time. Not too long... but long enough that they feel sure you've learned your lesson, and won't be so foolish again!
There is one catch though!
If your ship is a tech level of 10 or above... the Triarian Guard will NOT protect you in any way. Basically, if you have a tech level of 10 or more, they feel you are quite capable of protecting yourself if you want to get into a firefight, and will not use the guard's limited resources to give you a safe haven anymore. So, beware of that, if you intend on using them for any sort of shield or protection after you've past the basic levels of learning!