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:%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.