Evaluation Edition
The Evaluation Edition does not require a license. The Evaluation Edition is an unlicensed server, which provides full access to all features and options available in the Enterprise Edition. To run an unlicensed database beyond a 30-day evaluation period, however, you must purchase and install an appropriate license.
Express Edition
Unlike the Evaluation Edition, the Express Edition does not expire, but does place some limits on the way you deploy your server.