* Sill: A sill is a type of igneous intrusion that is horizontal or nearly horizontal. It forms when magma squeezes between existing layers of rock. It's important to note that a sill is not specifically related to a volcano. It can form in any geologic context where magma is present.

* Pluton: A pluton is a general term for any igneous body that crystallizes underground. Sills are a type of pluton. Plutons can also be dikes (vertical or near-vertical intrusions), batholiths (large, complex plutons), laccoliths (dome-shaped intrusions), and more.

Volcanoes and Intrusions:

* Volcanoes are formed by extrusive igneous activity, where magma erupts onto the surface as lava.

* Plutons are formed by intrusive igneous activity, where magma cools and solidifies below the surface.

While a sill can form near a volcano, it's not directly part of the volcano itself. The volcano's structure is primarily formed by the accumulation of lava flows, volcanic ash, and other materials erupted from the vent.

In Summary:

* Sills are flat, horizontal intrusions.

* Plutons are any igneous body that cools underground, including sills.

* Volcanoes are formed by extrusive activity and are separate from the intrusive activity that creates sills and other plutons.