What are the patterns and formations seen in selected area diffraction?

[Milchy]

Can anyone explain me the pattern that is obtained from selected area diffraction? What does it show? What are the dots? What are the rings? (example: http://www.microscopy.cz/html/images/2605_1_d0f65ce7_fig2.png)
I know that during X-ray powder diffraction, radiation is shined at the powder at varying incident angles. At certain theta angles the radiation diffracts from different lattices in the powder to form constructive interference, which shows up as peaks in the diffractogram. From the peaks you can calculate the latice spacing and miller indices.However, I am unable to understand/visually picture how the dots/rings are obtained from this TEM/diffraction method. Could someone explain me with some basic visual concepts using a unit cell and interacting/diffracting electrons and this shape formations?

 

[Lord Jestocost]

The rings are formed in a similar way as when performing X-ray diffraction on polycrystalline samples using the Debye-Scherrer method (http://pd.chem.ucl.ac.uk/pdnn/diff2/kinemat2.htm).