CREATE SEQUENCE orders_seq
 START WITH     1000
 INCREMENT BY   1;

Creates a sequence with a starting value of 1000 which is incremented by 1.

a reference to seq_name.NEXTVAL is used to get the next value in a sequence. A single statement can only generate a single sequence value. If there are multiple references to NEXTVAL in a statement, they use will use the same generated number.

NEXTVAL can be used for INSERTS

INSERT INTO Orders (Order_UID, Customer)
        VALUES (orders_seq.NEXTVAL, 1032);

It can be used for UPDATES

UPDATE Orders
SET Order_UID = orders_seq.NEXTVAL
WHERE Customer = 581;

It can also be used for SELECTS

SELECT Order_seq.NEXTVAL FROM dual;