libguestfs (GUEST FileSystem LIBrary) - the underlying C library that provides the basic functionality for opening disk images, reading and writing files, and so on. You can write C programs directly to this API.
guestfish (GUEST Filesystem Interactive SHell) is an interactive shell that you can use from the command line or from shell scripts. It exposes all of the functionality of the libguestfs API.
Various virt tools are built on top of libguestfs, and these provide a way to perform specific single tasks from the command line. These tools include virt-df, virt-rescue, virt-resize, and virt-edit.
augeas is a library for editing the Linux configuration files. Although this is separate from libguestfs, much of the value of libguestfs comes from the combination with this tool.
guestmount is an interface between libguestfs and FUSE. It is primarily used to mount file systems from disk images on your host physical machine. This functionality is not necessary, but can be useful.