To expand on Bjk's response... it's two sections that make or break an intake. The plenum, which is the area directly below the carb/throttle body and the port runner, which is the passage from the plenum to the head. Very basiclly, the smaller the plenum, and smaller and longer the runners, the more responsive and more low-rpm oriented and intake will be.
Single plane intakes were the original designs for most V style engines for years. The plenum is usually larger (at least in the performance designs), and the runners each feed one cylinder and all directly come off the plenum. The plenum allows the air/fuel mix to bend easier and enter each runner. However, too big is worse than too small in terms of plenum volume because when they are too large for a given package the air will slow down, the fuel can fall out of suspension, and the engine loses torque. This is why most single planes have the reputation for feeling sluggish. The plenums are larger and designed to move air better at faster rpm rates. So they will make more power over all in many cases, but the power curves tend to be more peaky, rather than flat or gentle curving toqrue curve of a street engine.
Dual planes combine ports that enter the plenum area, and they usually divide the plenum in half so the engine "feels" like it has a smaller plenum volume, however they also use larger runners to let it breath better at higher rpms and carry that power higher. The best dual plane factory style intake is the '71-'72 Thermoquads. Those are what the discontinued LD340 was designed after. They feed 4 cylinders, two from each side, into 1/2 of the plenum. Same design is now used in the Performer Edelbrocks, Stealths, and various others. The RPMs have a larger runner cross section, and a slightly larger plenum area to extend the power range up higher than the Performer with no loss of low end. Because the dual planes tend to idle better and make a flatter torque curve, they are the primary intake for the street. However, when you start looking at a complete pacakge, if a single plane intake ca work, it will almost always make more power. But the whole car has to be able to make use of the power they create in whatever rpm that happens.
