Extents are grouped together logically to form segments. They are the lowest level of logical storage in a database. Each user in Oracle has schema , which is a way of grouping database objects such as tables , indexes etc , so they are owned by that user and can be protected from other users. A user name and password is required to log on to a database. When a user is created an associated schema is created and the user has access to any of the objects in that schema.
Tables are fundamental unit of stroage of data and consist of rows and columns.
Each table in a schema must have a unique name and exits in tablespace. Indexes are schema object that is stored in its own segment. They are an alternative way of storing database tables. A view is a predefined way of looking at one or more joined tables which aer called base tables. A synonym is another name for an object.