SCADA vs HMI: what is the difference?
An HMI (human-machine interface) is the screen an operator uses to see and run a machine or process. SCADA is the whole system around it: it collects data from many PLCs, stores history, manages alarms, and serves HMI screens. Every SCADA system has HMIs; not every HMI is SCADA.
An HMI is a window. It might be a touch panel bolted to a machine, or a screen on a PC in the control room. It shows the operator what the equipment is doing (tank levels, motor states, alarms) and gives them buttons and setpoints to act on. A stand-alone HMI usually talks to one PLC or one machine.
SCADA is the whole system behind the windows. It polls many controllers, often at many sites, over networks and protocols such as Modbus and DNP3. It keeps a historian so you can trend last month's flows, and it runs alarm management across everything. It also serves HMI screens to operators wherever they are.
So the HMI is a part, and SCADA is the system it belongs to. A packaging machine with a touch panel has an HMI and no SCADA. A water utility watching forty lift stations from one control room has SCADA, and its operators use HMI screens to do it.
The line has blurred in modern software. A platform such as Ignition does both jobs: the same project can be a single machine's HMI or a plant-wide SCADA system with historian and alarming. The difference is still worth knowing, because job postings, quotes and specifications use the two words differently.