COUNT(*), COUNT(ALL expression), COUNT(DISTINCT expression)

Reference: https://msdn.microsoft.com/en-us/library/ms175997.aspx

COUNT(*) returns the number of items in a group. This includes NULL values and duplicates.

COUNT(ALL expression) evaluates expression for each row in a group and returns the number of nonnull values.

COUNT(DISTINCT expression) evaluates expression for each row in a group and returns the number of unique, nonnull values.

Example 

CREATE TABLE emp ( id INT );

INSERT INTO emp VALUES (1);

INSERT INTO emp VALUES (1);

INSERT INTO emp VALUES (2);

INSERT INTO emp VALUES (3);

INSERT INTO emp VALUES (null);

Table

Query

SELECT COUNT(*), COUNT(id), COUNT(Distinct id) 

FROM emp;

Result