What's the difference between ON voltage and threshold voltage in FETs?
By itself a FET does not simply have an ON state and an OFF state.
A FET has a "sliding" state depending on what voltages and currents you apply to it.

When \$Vgs\$ becomes larger than \$Vt\$ a conductive channel forms between drain and source. Does this mean there is no channel when \$Vgs &lt; Vt\$ ? NO it does not. \$Vt\$ is not a "hard" voltage, there is no sudden behavioural change (no discontinuity) when \$Vgs = Vt\$.

Regarding \$Von\$, it is what you define it to be. If I needed a LED to turn on and with my NMOS and it appears I need only Vgs = 1 Volt then I could say \$Von = 1 V\$.

Vt is a physical property of a MOSFET.

Von is a voltage you need to apply to the gate-source of a FET in order to get it in a certain state of conductivity (between drain and source). What this state is depends on someone's choice, not the MOSFET.

But a different Power MOSFET which controls a 100 A DC motor might need a Vgs of 10 V to turn ON then I would say \$Von = 10 V\$

Usually \$Von &gt; Vt\$ as you want the FET to be sufficiently "ON" so that it can conduct some current.