Blender has a strictly object oriented structure. Every aspect of the 3D world has been organized in small data blocks. By linking these together, making copies and edit or re-use them, you can build complex environments with a minimum of memory usage.  
Understanding this structure is very important for an efficient use of Blender.  

2.1.1 The main data blocks 

Scene. 
This is the 'canvas' of the 3D world. It contains the specific rendering information (camera, image resolution) and a the links to Objects. Different Scenes can use the same Objects. Scenes can also be linked together to function as a (film)set.  
World. 
This block contains sky, stars, exposure, and other environment variables.  
Object. 
The basic 3D information block. It contains a position, rotation, scale and transformation matrices. It can point to other Objects for hierarchies or deformation. Objects can be 'empty' (just an axis) or have a link to object data: Mesh, Curve, Lattice, Lamp, etc. Objects can be linked to animation curves (Ipo) and Materials.  
Mesh. 
This is the triangle and quad mesh data. It contains vertices, faces, and normals. It can have a keyframe block for morphing. It can be linked to Materials.  
Curve. 
Data to be used as text, Bezier or BSplines and 3D Nurbs Surfaces. It also has a keyframe structure and can be linked to Materials.  
Material. 
This data block contains visual properties like colors, reflectivity, transparency. It can be linked up to eight different Texture data blocks.  
Texture. 
Use images, procedures or plugins to define textures. Can only be linked to Materials and Lamps.  
Lamp. 
Data to be used for light information, as colors and shadow settings. Can be linked to Texture data blocks.  
Ipo. 
This is the main animation curve system. Ipo blocks can be used by Objects (for movement) but also by Materials.  

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