The basic difference between these two types of negotiation has to do with the "pie" that is being negotiated.
In distributive negotiations, the size of the "pie" is fixed. This makes this type of negotiation a zero-sum game. Whenever one side gains, the other side must necessarily lose. For this reason, this kind of negotiation tends to be very combative and unfriendly.
In integrative negotiations, the size of the pie is not fixed. It is possible for both sides to gain in such a negotiation. Therefore, negotiations of this sort can be less competitive and more cooperative.
Participants in a negotiation are not always clear as to which type of negotiation is going on. It can be difficult to tell if a situation is zero-sum or not. Therefore, it is possible to have different sides of a negotiation conceiving of the negotiation in different ways.