The document discusses various concepts relating to pixels in digital images including: - Pixel neighbors can be the 4 adjacent pixels or the 4 adjacent plus 4 diagonal pixels, called 4-neighbors and 8-neighbors. - Two pixels are considered connected if they are neighbors and their gray levels satisfy a similarity criterion. - Adjacency can be 4-adjacency, 8-adjacency, or mixed adjacency based on which neighbors are considered. Connected components are sets of connected pixels.