A mesh is a collection of indices and vertices. A vertex is a point in space and can have associated color, normal, uv coordinates and other vertex attributes. Indices are numbers that together with the topology of the mesh define primitives, such as triangles.
Simplified, you can think of the vertices as being dots on a paper with a number associated on them. The indices are a list which says which which vertices to use to build a primitive. Think of it as a connect the dots blueprint:
Except, usually we are connecting triangles or triangle strips instead of line strips in computer graphics. A mesh would define the “physical shape” of your building.
A texture is like a plain image, with some added properties. Images have wrap modes, filter modes and other settings that you can apply, such as mipmaps. For your house example, a texture would likely contain the color to paint all of your walls etc. It is what gives it “texture”. Hence the name. It is created with an image editing software, such as mspaint if you are a crappy artist like me, or photoshop if you are one of those rad artsy guys. Then it is imported into Unity.
A material is something that exposes properties for a given shader. A material by itself is not much more than a few values you can set and a reference to a shader. A shader is a piece of code that contain a vertex program and a fragment program. A shader is executed on the graphics cards GPU. The vertex program rearranges the vertices and the fragment program define the colors that will be output to screen. A shader uses vertices from the mesh and textures from the material.
Unity will render a mesh for you if you have a MeshRenderer component. It will use the mesh found on the MeshFilter component. You set the material on the MeshRenderer to specify what shader and which properties (textures, colors, other info) the shader will be supplied when executing on the GPU.
If i have a house and want the front to be different from the back and sides what do i use a texture, a material or a mesh.
It depends on how you want it to be different. Do you want the back to have a different shape? It would most likely be a different mesh. Do you want the back to have a different texture? You need either to update your texture or mesh in a mesh editing software such as maya, max or blender (I dont know what artists use, I am a coder) - to UV map that side to a new area on a texture. To be able to change material for the other side of the building, the other side of the building has to be a separate object.
I recommend you get a book on computer graphics and study it to learn it better.