SELECT 1 NUM_COLUMN, 'foo' VARCHAR2_COLUMN from DUAL
UNION ALL
SELECT NULL, NULL from DUAL;

NUM_COLUMN	VARCHAR2_COLUMN
1	foo
(null)	(null)

SELECT 1 a, '' b from DUAL;

A	B
1	(null)

SELECT 3 * NULL + 5, 'Hello ' || NULL || 'world'   from DUAL;

3*NULL+5	'HELLO'||NULL||'WORLD'
(null)	Hello world

SELECT a column_with_null, NVL(a, 'N/A') column_without_null FROM
  (SELECT NULL a FROM DUAL);

COLUMN_WITH_NULL	COLUMN_WITHOUT_NULL
(null)	N/A

NVL is useful to compare two values which can contain NULLs :

SELECT
    CASE WHEN a = b THEN 1 WHEN a <> b THEN 0 else -1 END comparison_without_nvl,
    CASE WHEN NVL(a, -1) = NVL(b, -1) THEN 1 WHEN NVL(a, -1) <> NVL(b, -1) THEN 0 else -1 END comparison_with_nvl
  FROM
    (select null a, 3 b FROM DUAL
     UNION ALL
     SELECT NULL, NULL FROM DUAL);

COMPARISON_WITHOUT_NVL	COMPARISON_WITH_NVL
-1	0
-1	1

If the first parameter is NOT NULL, NVL2 will return the second parameter. Otherwise it will return the third one.

SELECT NVL2(null, 'Foo', 'Bar'), NVL2(5, 'Foo', 'Bar') FROM DUAL;

NVL2(NULL,'FOO','BAR')	NVL2(5,'FOO','BAR')
Bar	Foo

SELECT COALESCE(a, b, c, d, 5) FROM 
    (SELECT NULL A, NULL b, NULL c, 4 d FROM DUAL);

COALESCE(A,B,C,D,5)
4

In some case, using COALESCE with two parameters can be faster than using NVL when the second parameter is not a constant. NVL will always evaluate both parameters. COALESCE will stop at the first non-NULL value it encounters. It means that if the first value is non-NULL, COALESCE will be faster.