Audio effects are systems designed to alter how an audio signal sounds. Audio effects can be either analog or digital, depending on which domain the audio is in. Unprocessed audio is referred to as dry, while processed audio is referred to as wet.
Historically, before the advent of widespread digital technology, analog was the only method by which to manipulate a signal. Since that time, as computers and software have become more capable and affordable and digital signal processing has become the method of choice.  However, some producers go to great lengths to incorporate analog effects in their music, as many people claim that it produces a "warmer" or more "authentic" sound than their digital counterparts. [citation] Regardless of whether an effect is analog or digital, most share a common set of controls and techniques which can be discussed and shared amongst all producers
Types of Audio Effects
- Equalization (EQ) - different frequency bands are cut or boosted to produce desired qualities. Moderate use of EQ can be used to "fine-tune" the quality of a sound; extreme use of equalization, such as heavily cutting a certain frequency can create more pointed effects. 
- Filtering- filtering is a crude form of equalization that cuts all frequencies above or below certain thresholds.
- Delay (Echo) - one or more delayed signals are added to the original signal. To be perceived as an echo, the delay has to be of order 35ms or above. 
- Reverb - essentially delay algorithms that create copies of the audio signal that get spread out over time and with varying amplitudes and frequency responses.  Reverb effects are used to mimic the sound of a hypothetical room of varying sizes (e.g. cathedral, hall, wooden room).
- Compression - the reduction of the dynamic range of a sound to avoid unwanted fluctuation in the dynamics.
- Limiting - prevents the amplitude of a signal from going over a given level.
- Gating - allows an input signal to pass through only if it is above a certain threshold.
- Overdrive - refers to a set of effects that produce a distorted sound. The most basic overdrive effect involves clipping the signal when its value exceeds a certain threshold.
- Flanger - a delayed signal is added to the original signal with a continuously variable delay (usually smaller than 10ms) 
- Phaser - a signal is split, a portion is filtered with an all-pass filter to produce a phase-shift, and then the unfiltered and filtered signals are mixed
- Chorus - a delayed signal is added to the original signal with a constant delay. The delay has to be short in order not to be perceived as echo, but above 5 ms to be audible. Often, the delayed signals will be slightly pitch shifted to more realistically convey the effect of multiple voices. 
- Autotune (Pitch Correction) - a musical signal is tuned to the correct pitch using digital signal processing techniques.
- Pitch Shifting - shifts a signal up or down in pitch.
- Resonators - emphasizes harmonic frequency content on specified frequencies.
- Modulation - changes the frequency or amplitude of a carrier signal in relation to a predefined signal.
- Burg, Jennifer; Romney, Jason; Schwartz, Eric .. "Digital Sound & Music: Concepts, Applications, and Science". Retrieved March 8, 2019.