An Oracle index-organized Table is a table stored in an index B-Tree structure. There is no second data structure (heap-table) for the table. The Oracle database always uses the primary key as clustering key. An Index-Organized Table is created with the ORGANIZATION INDEX clause:
CREATE TABLE ( id NUMBER NOT NULL PRIMARY KEY, [...] ) ORGANIZATION INDEXAccessing table data via a secondary index is slower than a similar query on a heap-table.
SQL Server supports index-organized tables as well, but uses the term clustered index.
Links
Book-Section: Index-Organized Tables
Glossary: Heap-Table — Tables stored in an unordered fashion.
Glossary: Secondary Index — Other indexes on a clustered index
