A version store is a collection of data pages that hold the data rows that are required to support row versioning. There are two version stores: a common version store and an online-index-build version store.
The version stores contain the following:
Row versions that are generated by data modification transactions in a database that uses snapshot or read committed using row versioning isolation levels. Row versions that are generated by data modification transactions for features such as online index operations, Multiple Active Result Sets (MARS), and AFTER triggers.
See http://msdn.microsoft.com/en-us/library/ms345368%28v=sql.105%29.aspx for more info.