In each of the examples until now, you've seen geometries and meshes being used. For instance, to add a sphere to the scene, we did the following:

var sphereGeometry = new THREE.SphereGeometry(4,20,20);
var sphereMaterial = new THREE.MeshBasicMaterial({color: 0x7777ff);
var sphere = new THREE.Mesh(sphereGeometry,sphereMaterial);

We defined the shape of the object and its geometry (THREE.SphereGeometry), we defined what this object looks like (THREE.MeshBasicMaterial) and its material, and we combined these two in a mesh (THREE.Mesh) that can be added to a scene. In this section, we'll take a closer look at what a geometry is and what a mesh is. We'll start with the geometry.