integrity constraint: DISABLE VALIDATE

in oracle 9i documentation i have read that DISABLE clause can be:
DISABLE NOVALIDATE (default. i understand vat it means) or
DISABLE VALIDATE:
disables the constraint and drops the index on the
constraint, but keeps the constraint valid. This feature is most useful in data warehousing situations, because it lets you load large amounts of data while also saving space by not having an index. All other modifications to the table (inserts, updates, and deletes) by other SQL statements are disallowed.
question1 if this means that data which violates the constraint is not allowed in the table, then vat is the difference between disable validate and an enabled constraint?
question2 is this only a 9i feature or is it available in oracle8 as well? because i dont find it in my oracle8 book (which we r following in school)

In short:

im sorry i must be particularly dense coz i still dont get it. vat does it mean by keeping the constraint valid?
also, is this only a 9i feature?

question 1:

It basically means that the index used to enforce the constraint is dropped and you can not modify the table data.
It's used to freeze the table for:
as you can read here which is also an example of the official docs coming with Oracle.
question2:
No, the feature is available in 8i also.