In SQL Server 2012, a new feature was added called Columnstore Indexes that resulted in huge query performance improvements. In SQL Server 2014, there have been two major improvements on this feature:
Clustered: They have enhanced the columnstore to be a pure columnar store, so indexing is no longer required. The In-Memory ColumnStore for data warehousing is implemented as a clustered columnstore index (or CCI) on a table. The data in a CCI is grouped and stored for every column in the table. Unlike the non-clustered columnstore index, the CCI is the data – there is no other underlying data structure
Updatable: You are able to insert, update, and delete data in an existing ColumnStore. Note that a columnstore index is impossible to update “in-place” due to its highly compressed structure, so “deltastores” are used. The same solution was done for v2 of PDW
Note you will still be able to create a non-clustered columnstore index which is not updatable. A table with a clustered columnstore index cannot have any type of nonclustered index.