the triode and saturation regions

Once a MOSFET is on, it can behave in two very different ways depending on the drain-to-source voltage. In the triode region (also called the ohmic or linear region) the drain voltage is small, and the device acts like a voltage-controlled resistor: current rises roughly in proportion to drain voltage, just like a resistor whose value you set with the gate. In the saturation region the drain voltage is larger, the channel pinches off near the drain, and the current flattens out, becoming nearly independent of drain voltage and set instead by the gate. Same device, two personalities.

The dividing line is when the drain-to-source voltage exceeds the gate overdrive (Vgs minus Vth). Which region you want depends on the job. A SWITCH lives deep in the triode region: you slam the gate fully on so the channel is a low resistance, and the small drain voltage means low loss. An AMPLIFIER lives in saturation: there the drain current is a clean, controlled function of the gate voltage, so a small gate wiggle produces a faithful current swing, the basis of the common-source amplifier.

Here is a notorious trap for newcomers. In a MOSFET, saturation is the GOOD amplifying region where current is constant; in a BJT, saturation means the transistor is slammed fully on as a switch. The same word means almost opposite things in the two devices. When someone says a MOSFET is in saturation they mean it is acting as a current source for amplification, not that it is a hard-on switch.