Components

Atmosphère consists of multiple components, each of which replaces/modifies a different component of the system:

Fusée: First-stage Loader, responsible for loading and validating stage 2 (custom TrustZone) plus package2 (Kernel/FIRM sysmodules), and patching them as needed. This replaces all functionality normally in Package1loader/NX Bootloader.
Exosphère: Customized TrustZone, to run a customized Secure Monitor
Thermosphère: EL2 EmuNAND support, i.e. backing up and using virtualized/redirected NAND images
Stratosphère: Custom Sysmodule(s), both Rosalina style to extend the kernel/provide new features, and of the loader reimplementation style to hook important system actions
Troposphère: Application-level Horizon OS patches, used to implement desirable CFW features

Licensing

This software is licensed under the terms of the GPLv2, with exemptions for specific projects noted below.

You can find a copy of the license in the LICENSE file.

Exemptions:

Nintendo is exempt from GPLv2 licensing and may (at its option) instead license any source code authored for the Atmosphère project under the Zero-Clause BSD license.

Credits

Atmosphère is currently being developed and maintained by SciresM, TuxSH, hexkyz, and fincs.
In no particular order, we credit the following for their invaluable contributions:

switchbrew for the libnx project and the extensive documentation, research and tool development pertaining to the Nintendo Switch.
devkitPro for the devkitA64 toolchain and libnx support.
ReSwitched Team for additional documentation, research and tool development pertaining to the Nintendo Switch.
ChaN for the FatFs module.
Marcus Geelnard for the bcl-1.2.0 library.
naehrwert and st4rk for the original hekate project and its hwinit code base.
CTCaer for the continued hekate project's fork and the minerva_tc project.
m4xw for development of the emuMMC project.
Riley for suggesting "Atmosphere" as a Horizon OS reimplementation+customization project name.
hedgeberg for research and hardware testing.
lioncash for code cleanup and general improvements.
jaames for designing and providing Atmosphère's graphical resources.
Everyone who submitted entries for Atmosphère's splash design contest.
All those who actively contribute to the Atmosphère repository.