Applies to: All units
Microphones can broadly be separated into three categories: Dynamic, Condenser and Ribbon.
The capsules found in Condenser (sometimes referred to as capacitor or electrostatic) microphones consist of a thin membrane (diaphragm) in close proximity to a solid metal plate. Sound pressure waves then move the diaphragm back and forth relative to the solid backplate. As the capsule is too fragile to connect directly to other audio gear and outputs almost no electrical current, active circuitry is needed to amplify the signal. Condenser mics require a DC power supply (transmitted through an XLR cable) in order to operate the active components used and therefore, a voltage (48V, but can range from 44V to 52V) is sent down the XLR cable to power the microphone. This voltage is commonly referred to as phantom power. Note that, even with phantom power on, the mic's signal will still need pre-amplification; the 48V supply is simply there to charge the condenser mic's capsule and power active electronic components.
Ribbon microphones use a thin metal 'ribbon' (commonly aluminium) placed between a pair of magnets in order to convert sound energy into electrical energy. In general, Ribbon microphones are passive so do not require phantom power. However, as an exception to this, there are some active ribbon microphones in production that do require phantom power. Passive Ribbon microphones (particularly vintage ones) can be severely damaged if phantom power is sent to them, so it is always advisable to contact the microphone manufacturer or check the user manual before connecting them.
As every engineer will tell you, best practice is to connect all your mics with phantom power turned off, then only engage phantom power on mic channels that need it once they are connected. If your mic preamp does not have a dedicated 48V button, there may be a global phantom supply switch, or phantom might be switchable in banks (say channels 1-4 and 5-8 on an eight-channel interface such as the Scarlett 18i20). When phantom power is supplied in banks, you should take care to ensure you do not connect equipment that could potentially be damaged by 48V being supplied, to an input within a bank being supplied phantom power.
Using phantom power with a Dynamic microphone should not damage the microphone but is not needed.
A Condenser microphone will not be damaged if phantom power is not engaged, though it will not pass any signal without it.
*The Scarlett CM25 (Bundled with the Studio Packs) is a Condenser microphone and will need Phantom Power.