1. Show all the lines that match a pattern, say PATTERN
:g/PATTERN


Use “g!” or “v” for negative matching. For example, to show all the lines that DO NOT match the pattern, PATTERN, use
:g!/PATTERN


2. Delete all the lines that contain the pattern, PATTERN
:g/PATTERN/d


Again, g! or v may be used for negative matching.

Application: You may use any of the following two commands to delete all the blank lines (including lines containing only whitespaces) from a file
:g/^\s*$/d
:v/\S/d


Reference: Vim Wiki.