Define Compound Keys Using Table Level Constraints

[Column level constraints go directly after the column definition to which they refer, table level constraints go after the last column definition.
Table level constraints are normally used (and must be used) for compound (multi column) foreign and primary key definitions (there would have been no difference to their function).

Let me explain this with example - Lets consider I am creating EMP table with EMPNO as primary key. As only one column is going to involve in constraint we can have column level constraint as well as table level contraint.
Create table EMP
(empno Number CONSTRAINT PK_Emp PRIMARY KEY,
ename varchar2(20),
deptno Number);

Or

CREATE TABLE EMP
(empno NUMBER ,
ename VARCHAR2(20),
deptno NUMBER,
CONSTRAINT PK_Emp PRIMARY KEY(empno));

Now consider if we want EMPNO and DEPTNO as primary key then we have to place primary key at table level.

CREATE TABLE EMP
(empno NUMBER ,
ename VARCHAR2(20),
deptno NUMBER,
CONSTRAINT PK_Emp PRIMARY KEY(empno,deptno));