ArchitectureWine implements the Windows API and application binary interface (ABI) in userspace, i.e. the region where most applications run on a typical operating system on top of low-level code like the kernel. This is done for security reasons, no thanks to Windows being susceptible to malware, but it comes at a cost of not being able to run certain games or applications which rely on strong Copy Protection or anti-cheat middleware such as StarForce, PunkBuster or GameGuard. Don't be surprised if the developers would deem a bug report as non-fixable, as they definitely wouldn't touch kernel-mode stuff with a ten-foot pole for the reasons mentioned above.
CompatibilityWine is able to run most Windows applications properly, especially older titles which may either malfunction or crash on a regular Windows system, and even old-school 16-bit applications on 64-bit *nix distributions, even if support for them has been dropped on Windows. This isn't to say that Wine is perfect though: it still is playing catch-up with Microsoft due to changes in the API, and as such newer technologies receive preliminary support or are yet to be implemented. For instance it usually takes a few years at most for a newly-released game to be functional on Wine, though some such as RAGE were reportedly playable almost immediately after its release.
DerivativesOwing to its open-source nature, Wine, or parts of it, has been used as the basis for numerous forks, such as Cider, a proprietary library/wrapper for developers to adapt existing Windows games to OS X with next to no changes in source code (as a matter of fact, this has been used by intrepid hackers to "port" various games and/or applications to Apple's operating system, simply by copying the game's installation folder to the Cider .app directory and tweak the configuration files if necessary), and CrossOver, a premium version of Wine developed by the same team behind the project. Aside from Crossover, there used to be another paid version of Wine called WineX (later rechristened to Cedega, and lastly as Game Tree Linux before being laid to rest) from TransGaming, the company who also developed Cider. The company eventually discontinued the product to focus on Cider. There is also a free Cider-like framework called Wineskin, allowing end-users and developers alike to do Windows-to-OS X conversions of software, without the legal issues associated with taking an existing Cider wrapper. Valve has also come up with their own fork, named Proton to allow Linux users to play Windows-exclusive titles without having to maintain a separate Windows partition or virtual machine. Proton also includes libraries from other Open Source projects, such as DXVK, a DirectX 10,11 and 12 translation layer that converts calls form those API's to Vulkan, which greatly improves the performance of games running under Proton.
One team even took Windows compatibility Up to Eleven by developing a free software alternative to Windows called ReactOS, which aims to be a drop-in replacement while still providing a familiar application and user experience to an average end-user without having to resort to any *nix-style OS. ReactOS shares much of its userspace code with Wine, saving them the trouble of having to redo everything from scratch. However unlike Wine, which is more or less feature-complete despite being in Perpetual Beta, ReactOS is still too unstable for daily use, either lacking crucial functionality or hanging up in certain scenarios. This is due to ReactOS' goal being more than just running Windows apps, but to become a complete drop-in solution for Windows XP/Server 2003 R2, including supporting all Windows' drivers, and having a fully custom kernel that is compatible with Windows system calls instead of just using the Linux kernel with a translation layernote . Additionally, certain Wine userspace libraries are not used by ReactOS either due to licensing, or due to incompatibilities with the way the other parts of ReactOS are implemented.note Unlike the developers of Wine, ReactOS developers considers low-level kernel issues a must-fix.