DBA_ROLLBACK_SEGS describes rollback segments.

Column Datatype NULL Description
SEGMENT_NAME VARCHAR2(30) NOT NULL Name of the rollback segment
OWNER VARCHAR2(6)   Owner of the rollback segment
TABLESPACE_NAME VARCHAR2(30) NOT NULL Name of the tablespace containing the rollback segment
SEGMENT_ID NUMBER NOT NULL ID number of the rollback segment
FILE_ID NUMBER NOT NULL File identifier number of the file containing the segment head
BLOCK_ID NUMBER NOT NULL ID number of the block containing the segment header
INITIAL_EXTENT NUMBER   Initial extent size in bytes
NEXT_EXTENT NUMBER   Secondary extent size in bytes
MIN_EXTENTS NUMBER NOT NULL Minimum number of extents
MAX_EXTENTS NUMBER NOT NULL Maximum number of extent
PCT_INCREASE NUMBER   Percent increase for extent size
STATUS VARCHAR2(16)   Rollback segment status
INSTANCE_NUM VARCHAR2(40)   Rollback segment owning Real Application Clusters instance number
RELATIVE_FNO NUMBER NOT NULL Relative file number of the segment header