Graphics and video
Variable refresh rate (VRR), sometimes also called dynamic refresh rate (DRR), refers to technologies that enables dynamic refresh rates for monitors, where the refresh rate of the monitor is continuously synced to the output rate of the content being displayed. This avoids any screen tearing, at least in the supported range, while also lowers power consumption as the monitor does not need to refresh at its highest supported rate while displaying content below that rate.
Variable refresh rate range (VRR range) refers to the range supported by a monitor where a dynamic refresh rate synced to the content is possible. This range usually starts at around 30 Hz and typically covers the full range upwards to the maximum refresh rate supported by the monitor.
VESA DisplayPort Adaptive-Sync and HDMI 2.1 Variable Refresh Rate are vastly different standards, but just about every practical considerations for variable refresh rate apply to both. The display must support the feature, an applicable cable must be used, and the output/source device (the GPU) must also support the feature and have it enabled. For the time being, the only GPUs with the capability of supporting the DisplayPort standard are AMD's GCN 2nd generation (e.g. Radeon HD 7790), Nvidia's GeForce GTX 10-series, or newer cards, while for the HDMI standard an AMD's Radeon RX graphics card or newer is needed.
While Intel do not currently have their own solution to allow variable refresh rate while using their graphics cards, in August 2018 they reconfirmed their commitment to support VESA DisplayPort Adaptive-Sync sometime in future GPU architectures.
FreeSync (or Radeon FreeSync as it is sometimes marketed as) is AMD's proprietary variable refresh rate technology and unique hardware/software solution built on top of the industry standards above, that it pushed itself to official specification in the first place. For example, FreeSync utilizes the VESA DisplayPort Adaptive-Sync protocols to allow dynamic refresh rates over a DisplayPort connection, and will also support the Variable Refresh Rate technology adopted in the HDMI 2.1 specification for HDMI connections. To enable the use of variable refresh rate technology over HDMI connections in monitors released 2016, almost two years before VRR became a part of the, at the time, future HDMI 2.1 specification, AMD and technology partners used vendor-specific HDMI extensions to implement AMD FreeSync over HDMI, thereby keeping the technology fully interoperable with HDMI standards.
While certified monitors also provide some really basic flickering guarantee, LFC (low framerate compensation) is only officially supported on monitors whose max/min refresh rates ratio is ≥2.5,  although it should work with anything just above 2x.
FreeSync 2 is a whole additional set of quality criteria that monitors needs to be certified and validated as supporting, ranging from mandatory LFC support to optimizations of HDR tone mapping process by allowing FreeSync to tone map the original HDR content of the game to the appropriate range of the display directly.
SetGPU is a DLL injector for DirectX 11 64-bit games which tries to force the use of an Nvidia graphics card of a system with the monitors connected to an AMD graphics card. This allows the use of AMD FreeSync with the Nvidia graphics card acting as a render card, while the AMD graphic cards is responsible for enabling and managing FreeSync.
G-Sync refers to Nvidia's proprietary variable refresh rate solution, hardware module, and related technologies and features available for use with some monitors with variable refresh rate capabilities when paired with an Nvidia graphics card. The use of G-Sync was restricted to only certified monitors up until 2019 when Nvidia announced an expansion of the G-Sync ecosystem to also allow the use of G-Sync on unlicensed monitors. Monitors with support for use with G-Sync can be categorized in four different groups: G-Sync, G-Sync HDR, G-Sync Compatible, and non-validated monitors.
G-Sync refers to the classic variant of Nvidia's variable refresh rate solution for certified monitors available in two forms depending on whether an external or integrated display is used. For external displays (sometimes seen as the "full" G-Sync experience) the typical internal scaler of the display is replaced with Nvidia's custom and proprietary G-Sync module. As a consequence an external G-Sync display does not have the built-in scaling support regular external displays typically have, and instead the display relies on the graphics card to scale the video signal as needed before sending the video signal over to the monitor. For integrated monitors (such as laptop monitors) a fundamentally different interface technology sometimes referred to as Mobile G-Sync is used instead. This technology does not require the proprietary G-Sync module, and instead the technology makes use of standardized embedded DisplayPort (eDP) features (e.g. VESA DisplayPort Adaptive-Sync) to enable dynamic refresh rates.
G-Sync HDR (also known as Ultimate G-Sync) is the HDR compatible G-Sync module, currently only available on a few expensive monitors. According to PC Perspective's findings when disassembling the ASUS ROG Swift PG27UQ monitor, the G-Sync HDR module alone might make up $500 of the final price point of G-Sync HDR compatible monitors.
G-Sync Compatible refers to VESA DisplayPort Adaptive-Sync compatible monitors that have been tested and validated by Nvidia to deliver a baseline VRR experience without flickering, blanking, or other forms of artifacts in VRR mode.
Non-validated monitors (not an official term) are VESA DisplayPort Adaptive-Sync monitors that either have not been tested and validated by Nvidia to be categorized as "G-Sync Compatible" yet, or was tested but found to not be able to deliver a validated experience. Users can still enable the use of G-Sync with these monitors by manually enabling G-Sync through the Nvidia Control Panel.
Nvidia Control Panel settings:
Enable G-SYNC for full screen mode.