Found an interesting article on fragmentation in SQL.

It explains the two types of fragmentation that can occur, namely internal and external fragmentation.
Internal fragmentation means that the pages are not completely full. While external fragmentation refers to the lack of correlation between the logical sequence of an index and its physical sequence.

For more details I’ll refer to this excellent blog post.

Source: Stairway to SQL Server Indexes: Level 11, Index Fragmentation – SQLServerCentral