From BlenderWiki

Contexts

The Buttons window shows six main contexts, which can be chosen via the first icon row in the header (see Contexts and Sub-Contexts Example). Each of these might be subdivided into a variable number of sub-contexts, which can be chosen via the second icon row in the header (same illustration), or cycled through by pressing the same context button shortcut again (see the Blender’s Contexts and Sub-contexts table below).

Unlike modes, contexts only concern the Buttons windows. And unlike modes, each Buttons window might be in a different context/sub-context.

Contexts and Sub-Contexts Example.

Blender’s Contexts and Sub-contexts
Contexts			Sub-contexts
Icon	Name	Shortcut	Icon	Name	Shortcut
	Logic	F4	None
	Script	None	None
	Shading	F5		Lamp	None
				Material	None
				Texture	F6
				Radiosity	None
				World	F8
	Object	F7		Object	None
				Physics	None
				Particle	None
	Editing	F9	None
	Scene	F10		Render	None
				Sequencer	None
				Anim/Playback	None
				Sound block	None

Once you have selected the context, the sub-context is usually determined by Blender on the basis of the active object. For example, with the Shading context, if a Lamp object is selected then the default sub-context is the Lamp one. If a Mesh or another renderable object is selected, then Material is the active sub-context, and if a Camera is selected the active sub-context is World.

The controls in each context are grouped into panels.