What are common walls of houses made of?

Drywall.

https://www.designingbuildings.co.uk/wiki/Drywall_construction

Most commonly, a wall delineates a building and supports its superstructure, separates space in buildings into rooms, or protects or delineates a space in the open air. There are three principal types of structural walls: building walls, exterior boundary walls, and retaining walls.

Building walls have one main purpose: to support roofs and ceilings. Such walls most often have three or more separate components. In today's construction, a building's wall will usually have the structural elements (such as 2×4 studs in a house wall), insulation, and finish elements, or surface (such as drywall or panelling).

https://en.wikipedia.org/wiki/Wall

A common wall is a wall common to two properties. It is therefore stress-bearing (supporting floors or rafters on both sides). Brick will be the commonest, with blocks of whatever they use to make them second, and stone third. The finishing is not the wall - it's what's inside that counts. Some may have the sound insulating properties of cardboard, but common walls are always made of solid material. https://en.wikipedia.org/wiki/Wall

Bit of UK based thinking there. Forgetting that a lot of houses in North America are made of tree rather than baked clay. I'm not counting concrete in, because the question says 'houses' and concrete is mainly used in flats rather than in houses as respects the walls.