GUI stands for graphical user interface, while HMI stands for human-machine interface. Despite their many similarities, these terms are often used interchangeably, but they actually refer to distinct types of interfaces. GUIs origin comes from the realm of personal computing, whereas HMIs have roots in industrial automation. However, a recent trend has emerged where automated systems integrate both types of interfaces within a single solution. 
The primary distinction between GUIs and HMIs lies in their respective scopes. GUIs are predominantly found in the personal computer (PC) sector, with only a small presence in industrial automation. However, this doesn’t diminish their relevance; GUIs can fulfill crucial functions within automated systems. In contrast, HMIs are largely exclusive to the realm of machinery and automation.
Furthermore, the operational principles differ between the two. HMIs, with their inputs and outputs linked to a Programmable Logic Controller (PLC), must closely adhere to the PLC program execution cycle. Conversely, GUIs operate on PCs, following distinct processes. Inputs to a GUI can trigger subroutine execution, activate separate applications, or prompt multi-threaded responses.
HMIs and GUIs represent distinct entities, although certain HMIs incorporate GUI functionalities, particularly those with touch screen capabilities. In highly automated systems, a combination of HMIs and GUIs is essential to achieve full operability.
