How to Calculate NDVI Using Landsat 8 Images in QGIS#15

The NDVI (Normalized Difference Vegetation Index) is a versatile vegetation index to evaluate the vegetation and dynamic of the vegetation coverage change. This index has been studied and evaluated with many satellites over the years, changing its calculus method in relation to the number and type of bands of the satellite

What is the significance of NDVI value from -1 to 1 in the interpretation of NDVI images?

Upon the classification of NDVI image in various ranges based on the NDVI value following significant information can be extracted from imagery.

Barren rocks, sand, or snow show very low NDVI values (-0.1 to 0.1)
Shrubs and grasslands or senescing crops – 0.2 to 0.5
Dense vegetation or tropical rainforest – 0.6 to 0.9
Deepwater- -1