These are modules that were specifically intended to sound like particular amp/cabinet combinations. Whether or not they sound like those particular amps, they are still worthy effects. Many of them might be considered overdrive or distortion effects.
Boosts usually increase the signal gain. The increase may be across the whole audio frequency spectrum, or just a portion of it. Some "boost" certain frequencies by cutting all others. Boosts may be clean, or may become overdrive or distortion effects as the gain increases.
Buffers are generally meant to reverse the bad effects of long cable runs and impedance problems. They are typically used to prevent loss of highs in long cable runs and signal chains. A restored signal may not be desirable in some cases, as with particular old fuzz effects.
Chorus is a slight delay and modulation effect that makes it sound like more than one guitar is being played, slightly out of time and tune with each other. At moderate levels, it provides a very pleasing thickening of the tone. At higher levels it can become "spacey".
The primary function of a compressor is to help manage the dynamic range of the notes played, sort of smooth out the volume of the individual notes. They can also impact sustain, frequencies, attack, and other aspects of the tone. Their impact can often be very subtle, although some provide a wide range of control that can be easily heard and felt.
Delays provide repeated echoes of the notes played.
Distortion is a crafted mangling of the input signal, often by clipping. Clipping may be direct and intentional or may result from high gain exceeding available headroom in some device. It is often similar in sound to overdrive, but more pronounced and more intentionally distorted rather than just boosted.
Envelope / Filter effects generally perform some type of modulation on some selected, perhaps sweeping, range of frequencies.
An equalizer divides the audio frequencies into several frequency bands that can each be cut or boosted individually. Often there is also an overall volume cut/boost control. They are similar to simple Bass/Mids/Treble tonestacks, but provide a better level of control over specific frequency bands.
GT enclosures can hold up to 17" (horizontally) of effects modules, mostly in 1.5" and 2" module increments. The entire 17" of the enclosure must be filled. Filler panels are "blank" front and rear panels that fill the unused space.
Flangers are similar to chorus effects. They cause a duplicate of the signal to be played back and mixed with the original signal with a short delay and slightly out of phase. They can be very subtle ambient effects or big, lush, jet "whooshes".
Fuzzes come in a great variety of sounds and tones, defying general description other than usually the most distorted of distortions, although some can also be used as nearly-clean boosts. Often they produce nearly square waves, many with additional overtones added.
Overdrives were meant to be stronger boosts that would cause something else in your signal chain to begin distorting. However, some add considerable distortion themselves, blurring the distinction. It is sometimes convenient to think of them as one part of a boost - overdrive - distortion - fuzz continuum.
Phasers are similar to flangers. They take a copy of the input signal, put it out of phase, and combine it back with the original signal, creating peaks where the phases of the two signals align and drops where the phases of the two signals are opposite. There are multiple "stages" of phase changes, providing multiple peaks and drops, usually varying through different frequencies. The result is a "sweeping" effect that can be subtle and ambient or strong and "spacey".
Modules that provide the DC power for the other modules
Reverb modules provide a simulation of the natural echoes and reverberation that occur when sounds bounce around in an enclosed space and give your tone spatial dimension.
Routing modules are not effects. They help route your signal in various ways through your chain. Loopers, mixers, ABY selectors, and others allow you to create more complex effects setups.
Switching modules are GT-specific modules used to control the switching of other modules. This can be simple remote on/off switching, or more complex "tone switching".
Tremolo modules modulate the volume of your signal, causing the volume to rise and fall at amounts and rates you can control.
Vibrato modules modulate the pitch of your signal, causing the pitch to rise and fall at amounts and rates you can control.
Bass modules aren't a category like the other categories in this list. Instead of being grouped by what they do, they are grouped here by how they are used. All the bass effects listed will be links to pages that have information about module configurations or mods that are meant for bass. Some of the modules are specifically designed for bass, and some are guitar modules made "bass friendly" with customizations. There are undoubtedly more guitar modules that might work fine for bass, including some of our delay, flanger, reverb, phaser, and tremolo modules. Probably also some overdrive, distortion, and fuzz modules. We don't list them all here. Take the listings in this category as guidelines to get you going.