I am trying to download NDVI raster images but I am quite confused as to how many different types are available. I see there is a NDVI, MODIS NDVI, MODIS 16-day/32 days, a Filled NDVI and all at different resolutions, are TERRA and MODIS the same thing? Can someone make some clarity? 
Why all these different websites where to download, and is it the same product?
http://glcf.umd.edu/data/modis/http://glcf.umd.edu/data/ndvi/http://neo.sci.gsfc.nasa.gov/view.php?datasetId=MOD13A2_M_NDVIhttp://pekko.geog.umd.edu/usda/test/http://glcfapp.glcf.umd.edu:8080/esdi/index.jsphttp://land.copernicus.eu/global/products/ndvihttp://earthexplorer.usgs.gov/
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Why all these different websites where to download, and is it the same product?http://glcf.umd.edu/data/modis/http://glcf.umd.edu/data/ndvi/http://neo.sci.gsfc.nasa.gov/view.php?datasetId=MOD13A2_M_NDVIhttp://pekko.geog.umd.edu/usda/test/http://glcfapp.glcf.umd.edu:8080/esdi/index.jsphttp://land.copernicus.eu/global/products/ndvihttp://earthexplorer.usgs.gov/
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