:%s/foo/bar/g        Find each occurrence of 'foo' (in all lines), and replace it with 'bar'. 

:s/foo/bar/g         Find each occurrence of 'foo' (in the current line only), and replace it with 'bar'. 

:%s/foo/bar/gc       Change each 'foo' to 'bar', but ask for confirmation first. 

:%s/\<foo\>/bar/gc   Change only whole words exactly matching 'foo' to 'bar'; ask for confirmation. 

:%s/foo/bar/gci      Change each 'foo' (case insensitive) to 'bar'; ask for confirmation. 
                     This may be wanted after using :set noignorecase to make searches case sensitive (the default). 

:%s/foo/bar/gcI      Change each 'foo' (case sensitive) to 'bar'; ask for confirmation. 
                     This may be wanted after using :set ignorecase to make searches case insensitive. 

Search and replace