A phased implementation is one where, at go-live, only certain critical functionality is available to staff and customers. For example, a phased implementation might include membership and committee management, but not events management. Events management would come after go-live.
Because phased implementations are more focused (i.e., fewer pieces of functionality required at go-live), they can be very effective for getting to go-live more quickly.
But the downside of phased implementations is that, if not managed very tightly, they can go on forever. A good phased implementation will have all phases clearly defined, including delivery dates for each phase. And if the scope changes (as it invariably will), then the delivery dates and items need to be modified, as well.
So be careful with phased implementations, because if they are not scoped out and strictly managed, they may never end!