Headphone Driver types
There are different types of drivers. They all have different properties and is the reason for their existence.
The smallest ones are the BA (balanced Armature) drivers. They are found in (C)IEM’ but not all IEM’s have BA drivers, they might as well be equipped with dynamic drivers. (C)IEM’s provide an excellent attenuation from outside noises and is why you often see musicians/singers wearing CIEM’s as monitors during their performance. Some IEM’s have a single driver, others can have 2 or more connected via a very simple crossover network. As these drivers are very small the diaphragm can be very small and lightweight which can make them ‘fast’. They drivers work on magnetic principles. 2 small fixed magnets are situated at the end of a thin metal U shaped form called the armature. A small coil is surrounding the metal armature which is magnetised by the coil (when an audio current flows).
This metal armature subsequently is pulled in the direction of either magnet depending on the polarity of the signal on the coil. That up and down movement is transferred by a small rod to a light weight membrane (diaphragm) which moves in the same way as the armature. This movement creates a sound pressure in the sound chamber and the sound leaves the driver through a small hole into little ‘channels’ leading to the earpiece. The drawing shows a basic form and plenty variations exist.
Most headphones, however, have dynamic drivers with a voice coil and a small membrane. They exist in many different sizes ranging from very small inside IEM’s to large diameter ones up to 55mm. They consist of a ring magnet and metal yokes that ‘conduct’ the magnetic forces and concentrate them at the spot where the voice-coil is situated. The voice-coil is 
made of very thin wire, thinner than a hair, and wound in a coil. This coil is wound on a lightweight former that is in turn connected to a (mostly dome shaped) diaphragm. When current flows through the voice-coil it created a magnetic field that repels the diaphragm outwards and inwards depending on the polarity of the audio signal. The drivers exist in many sizes, shapes and the diaphragms can be made of different types of materials and or may have coatings. The material the diaphragm is made of (together with the size and shape) determine the sonic properties of the driver.
Another type of driver is the ortho-dynamic driver or ISO-dynamic driver which is a flat (planar) membrane sandwiched between strong magnets with holes (space) between them and a coil etched on the membrane that is stretched tightly. This coil can have 
very different shapes and number of ‘windings’ depending on the arrangement of the magnets. The membrane itself can be very thin but the coil material on that membrane always adds a substantial percentage to the weight. The efficiency of these drivers depends on the strength of the magnets, the distance to the membrane and the number of windings as well as the current (voltage/impedance). In general the efficiency is lower than those of dynamic drivers meaning with the same amount of electrical energy applied (Wattage) the dynamic drivers will play louder. A big advantage of the ortho-dynamic drivers is that the membrane is moving as a whole as the driving forces are the same all over the membrane where as in dynamic drivers only the point where the voice-coil former connects to the diaphragm is ‘driven’. A variation of this principle exists where the membrane only has 
magnets on one side of the membrane. These drivers, however, do not have an equally good linearity. The linearity of normal planar magnetic drivers is very high. Because of this property very high SPL can be reached without much distortion.
A variation of this type of planar drivers is the electrostatic driver and the electret driver. An electret has a membrane with a static charge on it sandwiched between 2 perforated metal plates (stators) and is the ‘cheap’ cousin of the electrostatic type which requires an additional bias voltage. The electret only has 2 connections, being the plates (stators) on both sides.
The electrostatic driver is basically similar to the electret BUT the membrane doesn’t have a ‘pre-charge’ but is connected to a (rather high) bias voltage. This voltage can be between 100 and 400V or even higher depending on the distance between the membrane and plate. Larger distances = larger swings of membranes and thus more SPL and more lows (depending on mechanical tension as well).
It works on the same principle as the balloon that is rubbed with wool. An electrostatic force is present on the balloon which attracts hairs or can ‘stick’ to other materials. On the 2 stators an electrical signal with a high voltage (up to several hundreds of volts !) is applied. Each stator has an opposite voltage so if one stator has +130V on it the other one has -130 Volt on it. The very thin membrane (that is much lighter than an ortho dynamic because it doesn’t have a conductive coil on it) also has a high voltage on it and is attracted to one stator and repelled from the other depending on the polarity and magnitude of the audio signal. Similar to the orthodynamic but using electrostatic principles using voltages instead of currents and magnetic principles. As the membrane is driven over it’s entire surface AND is very light-weight it is the king of transients. Meaning the membrane follows fast electric signals very fast and accurate where heavier membranes are ‘slowed down’ by their weight.
The electret is usually driven with an energizer (transformer) that is connected to a loudspeaker amplifier and cannot be driven 
well from a headphone amplifier or portable equipment. Electrostatic headphones are driven by either a similar energizer, but also provides the bias voltage or by a dedicated amplifier that provides the very high voltages needed. Stax even makes an earbud type electrostatic earphone with a portable energizer/amplifier. Most headphones only have 1 driver that covers the entire frequency range but some (C)IEM’s have multiple drivers with crude crossover filters somewhat similar to those found in speakers. The AKG K340 (the older one with this type number not the earbud with the same typenumber) has dual drivers, one electret and a dynamic driver. Some very rare and exotic transducers (drivers) are bone-conduction drivers and air-motion transformers.
DIY-Audio-Heaven 
What is the purpose of the tape across the holes on a dynamic driver? Should it be left in place or should it be removed and under what circumstances? If removed how would you do it?
The purpose is damping of the membrane.
Removing it usually results in more midbass an poorer sound.
Drivers headphones that already have low amounts of bass will not get more lows.
That paper is there for a reason and is specifically tuned so the driver works well with the enclosure it is in and the pads.
One can experiment with temporarily sealing the holes (tape) or part of the holes.