Hi, this is Oracle Girl. In the previous lesson we learned about the relational database and how the object structures are related. In today’s lecture we will see how those relationships are represented in Oracle RDBMS. The Oracle Database architecture consists of three major components: memory, processes, and storage. We will have a look at the storage logical structures.
The smallest unit of storage in Oracle is block. It consists of one ore more operating system blocks. The proper size of it, is decided during database creation. This parameter cannot be modified afterwards.
The next level of logical grouping is called extent. It is a group of continuous blocks, allocated in one chunk.
Set of extents makes up a Segment – the next level of logical storage structure.
Whenever you create a logical structure that stores data such as table, Oracle allocates a segment, which contains at least one extent.
The segments are grouped in the highest level of logical structures called tablespace. The tablespaces have a direct relationship to the physical structure of the database – datafile. We will discuss the physical structure in our next lesson.
Let’s summarize. Looking at the logical structure of the database, a tablespace is the highest level of logical unit. A tablespace consists of several segments. A segment consists of one or more extents and can be associated to only one tablespace. An extent is a contiguous allocation of blocks. A block is the smallest unit of storage in an Oracle database.
Thank you for your participation in today’s lesson. Please let me know if you have questions or recommendations in the comment section below.
You can see this as video lesson 
resources:
OCA: Oracle Database 12c Administrator Certified Associate Study Guide, Sybex
Official Oracle Documentation 
Talk to you soon, Oracle Girl