The answer depends on how one defines the term "database" - there is not single, universal definition that all agree on.
Xcalar is unique and different when compared with most Database Management Systems because Xcalar does not store its own persistent copy of the data. Instead Xcalar imports data directly from external systems where the data originates. The benefit here is that you don't end up with another copy of the data, and you don't lose visibility to any transformations as the data is moved to Xcalar and processed by Xcalar.
Xcalar offers relational processing on arbitrary (ie: almost any) data - so that is functionally similar to Relational Database Systems, however Xcalar can typically deal with much more diverse varieties of data, and much larger volumes.