concepts and terms

a song (also called module) is a file for a tracker that contains at least one subsong.
each Furnace song involves at least one chip, an emulation of a specific audio processor.

the pattern view is similar to spreadsheet that displays the following:

each labeled column represents a channel of sound provided by the chips in use.
each note starts a sound playing. within a channel, only one note can play at a time.
each note is assigned an instrument which describes what it will sound like.
an effect is a command that changes some aspect of playback. it can alter note pitch, volume, timing, and more.
an instrument macro is an automated sequence of effects that applies to every note of that instrument.
during playback, the playhead moves down, scrolling through the pattern view, triggering the notes that it encounters.

the order list is a smaller spreadsheet showing the overall song structure.

a song is made up of a list of orders.
an order is a set of numbered patterns used for each channel.
each channel has its own unique list of patterns.
each pattern contains note and effect data for that channel only.
patterns may be used multiple times in the order list. changing a pattern's data in one order will affect the same pattern used in other orders.
each pattern is made of the same number of rows as seen in the tracker view.
during playback, the playhead moves down as described previously. when it reaches the end of the pattern view, it will go to the next order.
if the last order is reached and the playhear reaches the end of the pattern view, it will go back to the beginning of the song.

during playback, each row lasts a number of ticks determined by the song's speed value(s).
a tick is the smallest measure of time to which all note, effect, and macro times are quantized.

sound chips have different capabilities. even within the same chip, each channel may have its own ways of making sound.

some channels use one or more waveform generators (sine, square, noise...) to build up a sound.
of special note are FM (frequency modulation) channels, which use a number of generators called operators that can interact to make very complex sounds.
some channels use samples which are (usually) recordings of sounds, often with defined loop points to allow a note to sustain.
some channels use wavetables, which are very short samples of fixed length that automatically loop.