http://www.oraclefaq.net/logical-structure-of-database/
Logical structure mainly consists of tablespace, segments, extents, and oracle data blocks
TABLESPACE
Each database is logically divided into one or more tablespaces. One or more datafiles are explicitly created for each tablespace to physically store the data of all logical structures in a tablespace. The combined size of the datafiles in a tablespace is the total storage capacity of the tablespace.
SEGMENT
A segment is a set of extents allocated for a certain logical structure. The segments can be of one of following type data segment,index segment,temporary segment,rollback segment.
EXTENT
The next level of logical database space is an extent. An extent is a specific number of contiguous data blocks, obtained in a single allocation, used to store a specific type of information.
ORACLE DATABLOCKS
At the finest level of granularity, Oracle database data is stored in data blocks. One data block corresponds to a specific number of bytes of physical database space on disk. The standard block size is specified by the DB_BLOCK_SIZE initialization parameter. In addition, you can specify up to five other block sizes.