Extents and segments
From a physical point of view, a datafile is stored as operating system blocks. From a logical point of view, datafiles have three intermediate organizational levels: data blocks, extents, and segments. An extent is a set of data blocks that are contiguous within an Oracle datafile. A segment is an object that takes up space in an Oracle database, such as a table or an index, that is comprised of one or more extents.
When Oracle updates data, it attempts to update the data in the same data block. If there is not enough room in the data block for the new information, Oracle will write the data to a new data block, which may be in a different extent.