A detailed explanation of Pre vs Post Clickstream columns can be found in an update KB article as well:
http://microsite.omniture.com/t2/help/en_US/sc/clickstream/datafeeds_column_types.html
Pre and Post Columns
The pre column contains the data as it was sent to data collection. The post column contains the value after processing.
For example, variable persistence, processing rules, VISTA rules, and currency conversion might change the final value recorded for a variable that appears in the post column. For most calculations you want to use the post column unless you are applying custom business logic (for example, applying a custom formula to determine attribution).
If a column does not contain a pre or a post version (for example, visit_num), then the column can be considered a post column. Columns prefixed with "pre_" typically contain data that was populated by Adobe and not sent by your implementation. For example, pre_browser is populated by Adobe, but evar1 is populated by your implementation. Both of these columns have a "post_" column (post_browser, post_evar1), which contains the value used by reporting.
There are several reasons that this could happen. Even though we treat these variables as case insensitive for reporting purposes, if customers pass in mixed case values we try to preserve the case for reporting since our customers have told us that they want it done this way.
Best,
Brian