SPListItemVersion class

Use the Versions property of the SPListItem class to return the collection of versions for a list item. If versioning is enabled on a list, each time you modify an item a version is created.

Use an indexer to return a single version from the collection. For example, if the collection is assigned to a variable named myListItemVersions, use myListItemVersions[index] in C#, or myListItemVersions(index) in Visual Basic, where index is the index number of the version in the collection.

To return a specific column value for a list item version, use an indexer on the version. For example, use myItemVersion[index], or myItemVersion(index) in Visual Basic, where index is either the index number of the field in the collection of fields associated with the list, or the name of the field.

Reference