Hybrid is a broad label covering layouts that work in quite different ways. The most useful division is whether the combustion engine has a mechanical path to the wheels at all.
What a series hybrid does
In a series arrangement the engine is not connected to the wheels. It turns a generator, and the electricity produced either drives the motor or charges the battery.
The car is therefore always driven electrically, and the engine functions purely as an onboard power station.
Because the engine has no obligation to match road speed, it can run at whatever speed and load is most efficient, or shut down entirely.
Why series loses out at steady speed
Energy passing through a series system is converted twice, from mechanical to electrical at the generator and back to mechanical at the motor.
Each conversion loses a little, and at a constant cruise those losses accumulate against a direct mechanical connection that would have avoided them entirely.
This is why pure series layouts are strongest in stop-start driving and weakest on a long motorway run.
What a parallel hybrid does
In a parallel arrangement the engine drives the wheels mechanically, and an electric motor is added to the same driveline to assist it.
At a cruise the engine drives the car directly with no conversion losses, and the motor contributes during acceleration when demand is highest.
The motor also recovers energy under braking, and can allow the engine to be switched off during coasting and low-load running.
Why most systems now combine both
Power-split designs use a planetary gearset and two electric machines to divide engine output between a mechanical path and an electrical one continuously.
At low speed most of the power travels electrically, which suits urban conditions. At higher speed more travels mechanically, which suits cruising.
Other manufacturers achieve a similar result by running as a series hybrid in town and engaging a single direct-drive clutch at higher speed.
What this means for how each drives
A series-dominant car feels electric, with the engine note unrelated to road speed, which some drivers find disconnected until they understand why.
A parallel system feels closer to a conventional automatic, since engine speed still tracks what the car is doing.
Neither is inherently superior. The right choice follows from how the car is used, which is why the same manufacturer may sell both layouts in different models.
The practical consequence is that two cars described identically as hybrids can behave nothing alike, and the label on the boot lid tells a buyer far less than the layout underneath does.