Laurentia is another name for the North American craton, the large and very old chunk of rock that North America rests on. The "basement complex" — the metamorphic and igneous rock below the sedimentary layer — of Laurentia is 1.0 to 3.0 billion years old, and was created in a tectonically active setting, under great pressure and temperature. Its constituent rocks are all igneous oxides, like granite. Like the other continents, Laurentia gets pushed around by ocean spreading, bumping into them and occasionally forming supercontinents like Pangaea.
This landmass is named after the Laurentian craton, which is named after the St. Lawrence river, which flows over it. The Laurentian craton underlies the entire North American continent, but only reaches the surface in northern Canada, where the sedimentary rocks were scraped clean off during the last Ice Age. The Laurentian craton is the world's largest exposed area of Archean (older than 2.5 billion years) rock. It features numerous geologic spectacles, including the Mackenzie dike swarm, a 311 mile (500 km) wide and 1,864 mile (3,000 km) long track of cooled magma from a prodigious eruption 1.2 billion years ago, and the world's oldest volcanoes.
Over the hundreds of millions of years, Laurentia's shape has changed slightly, due to bumping into other cratons; volcanic activity, especially large igneous provinces, which may be extruded over the course of a million years; and sedimentary accretion. At the margins of the continents are where much of the mountain-building takes place, due to pressure between colliding cratons. The Laurentian craton has mountain ranges on both margins — including the Sierra Nevadas in the west and the Appalachians in the east.
Part of the reason the name "Laurentia" is necessary is that the continent has been in various different configurations throughout its history, and is not necessarily always "north" of anything, making "North America" inappropriate. Laurentia has been a component of the supercontinents Kenorland, Nena, Columbia, Rodinia, Protolaurasia, Pannotia, Euramerica, Pangaea, Laurasia, and the current minor supercontinent, America. For tens of millions of years, during the Cretaceous period, Laurentia was divided down the middle by the Western Interior Seaway. Various marine fossils can be found in the central USA and Canada because of this.