rowtotal -> Reports an estimate of the number of rows. The value is read from the OAM page. Though not always exact, this estimate is much quicker and leads to less contention than select count(*).
reserved -> Reports pages reserved for use by the table and its indexes. It includes both the used and unused pages in extents allocated to the objects. It is the sum of data, index_size, and unused.
data -> Reports the kilobytes on pages used by the table.
index_size -> Reports the total kilobytes on pages used by the indexes.
unused -> Reports the kilobytes of unused pages in extents allocated to the object, including the unused pages for the object's indexes.
Hope this will be helpful.
~Gaurav