Crust
The crust is Earth's thin, rocky outermost layer, divided into thicker granitic continental crust and thinner basaltic oceanic crust.
Definition
The crust is the thin outer shell of rock that we live on, and compared to the whole Earth it is thinner than the skin on an apple. It is Earth's outermost solid layer: continental crust averages about $35$ to $40\ \text{km}$ thick, is made mostly of granite-like rock, and is less dense (about $2.7\ \text{g/cm}^3$), while oceanic crust is about $7\ \text{km}$ thick, made of basalt and gabbro, and denser (about $3.0\ \text{g/cm}^3$). The boundary between crust and mantle is the Mohorovicic discontinuity, or Moho, defined seismically where P-wave velocity jumps from about $6$ to $7\ \text{km/s}$ to about $8\ \text{km/s}$ as feldspar-rich rock gives way to peridotite. Continental crust is chemically evolved, averaging andesitic to granodioritic composition with about $60\%$ $SiO_2$, and thickens to $70\ \text{km}$ under the Himalaya and Tibet. Oceanic crust forms at mid-ocean ridges with a layered structure of pillow basalt, sheeted dikes, and gabbro over mantle peridotite (the ophiolite sequence). The crust makes up less than $1\%$ of Earth's mass.
Example
Everything you can see, from the Charlotte skyline to the Appalachian Mountains to the ocean floor, sits on the crust. Under the Appalachians the continental crust is roughly $45$ to $50\ \text{km}$ thick, like the deep root of an iceberg, while under the Atlantic seafloor east of the Outer Banks the oceanic crust is only about $7\ \text{km}$ thick. Isostasy explains crustal elevation: like ice floating on water, thick, low-density continental crust rides high, and mountain roots extend deep into the mantle. The crust under Scandinavia is still rising several millimeters per year as it rebounds from the weight of Pleistocene ice sheets.
Key Insight
The crust under the oceans is thinner and heavier than the crust under the continents, and that is part of why the oceans sit lower. Oceanic crust is young, none older than about $200$ million years, because it is recycled by subduction, while continental crust is too buoyant to sink and includes rock $4$ billion years old. The crust is not the same as a tectonic plate. Plates are lithosphere, which includes the crust plus the rigid uppermost mantle, and the two layers move together.