A Base of Operations serves as the home of an Irken Invader and his/her SIR Unit while stationed on the planet they intend to conquer. Irken Invaders are given a small device where they can draw the base they wish to have, and then the device creates it. In the base, the Irken Invader and his or her SIR Unit plot to conquer their assigned planet, and enslave its native race, or, if it is not sapient, wipe them out.
Zim's Base is underneath his home, where he plots to wipe out mankind. Tak's base is inside the City's Deelishus Weenie factory. Invader Tenn´s base has been seen on planet Meekrob, although it was exposed by many malfunctioning SIR Units. Invader Slacks' base was briefly seen on Boodie Nen, Invader Larb's base on Vort presumably had the Universe's Most Comfortable Couch in it and Invader Skoodge´s base although unseen, was on planet Blorch.
While an Invader can draw out the basic outline of what their base should look like, the computer within usually does all the work. In "The Nightmare Begins", Zim drew out his base, but the color scheme of the house and the whole of the inside of the base created itself. It's unknown whether this is a unique kind of technology Zim has, or if all Invaders have this, however seeing how the Tallest have a strong hatred for the would be Invader, its more than likely that the tech Zim received from his leaders to build his base is an outdated model of the standard Invader equipment.
Zim's base is defended by his Lawn Gnomes and Roboparents. The lawn gnomes have the ability to shoot painful lasers at enemies, as seen in "Zim Eats Waffles", or drag enemies out like in "Gaz, Taster of Pork". The Roboparents are supposed to resemble human parents, but in actuality they act more like aliens than Zim; for example, they once asked the neighborhood to come help them eat a child.