There are 11 main parts to the LMMS interface. These are described in detail in their respective pages.
The Main Menu Bar serves as a general menu for LMMS.
The Toolbar contains options for managing project files, showing/hiding interface windows, and altering master controls for projects.
The Sidebar is used to introduce resources to a project.
A majority of the interface contains a number of windows, each serves a different purpose.
The Instrument Window contains settings for an instrument.
The Song Editor contains all the different sound-generating elements and automation clips in a project file.
The Beat+Baseline Editor is where repetitive loops (such as drum/basslines) can be created and contained.
The Piano Roll allows you to create and edit note sequences.
The Automation Editor allows you to create a profile that any connected parameter can follow.
The FX-Mixer is where FX channel routing and manipulation takes place.
The Project Notes window allows you to create notes for yourself in a project file (For example, "B+B is only to be used for drumlines in this project".)
The Controller Rack is where controllers are added/configured, these controllers allow you to make alterations to connected parameters (like an Automation Clip would).