Good question…
We’re actually going to look at two definitions: sequencer and synthesizer, because one cannot exist without the other to produce sound.
Let’s use an image to illustrate this. Imagine a pianist. He reads a score, converts it in his head into notes, durations, nuances, dynamics, tempo, and sends his fingers the commands to play, which cause him to place his hands and fingers in the right place at the right time so that the piano produces the desired sound.
We can see that there are two distinct elements: the one who reads the score and sends the commands, and the one who produces the sound according to the commands it receives.
The pianist would be the sequencer and the piano the synthesizer….
Some definitions gathered from the internet:
What is a sequencer?
A sequencer is a device or software used to record, edit, and play back sequences of music or data.
It organizes information such as MIDI notes or control signals into patterns or tracks.
Sequencers are essential to music production because they offer precise synchronization and automation capabilities.
What is a synthesizer?
A MIDI synthesizer is a device or software that synthesizes sound in response to incoming MIDI data.
In its simplest form, a MIDI synthesizer converts MIDI messages into an audio signal.
More complex synthesizers offer precise control over the sound produced, allowing instruments to be imitated.
So, on the left is the sequencer (our pianist) and on the right is the synthesizer (his piano).
The reality is obviously more complex than our simple example. As in our example, the sequencer can send its execution commands to the synthesizer (PLAYBACK) to play an existing piece, but the “opposite” is also possible: what is played on the synthesizer by a pianist improvising, for example, can be recorded in the sequencer (RECORDING) to create a new piece of music.
The sequencer can therefore play an existing piece (a sequence) on the synthesizer, but also record a new sequence (which would be played on the synthesizer)
In reality, the sequencer can be a physical device or software (such as Karaboss).
In the case of software sequencers, it is possible to enter notes into the software interface.
The synthesizer can be an electric piano with a MIDI input (MIDI IN) for performance and an output jack for recording (MIDI OUT), but also a digital keyboard capable of synthesizing any sound, and even software (such as VirtualMidiSynth).
Therefore, each of these two devices can be purely software-based (the Karaboss sequencer + the VirtualMidiSynth synthesizer), purely hardware-based (a physical sequencer and a digital keyboard), or a combination of the two: a software sequencer such as Karaboss and a digital keyboard.