The Arab Spring refers to the current crop of pro-democracy uprisings currently (2011) sweeping the Middle East and North Africa. The "spring" is in contrast to the "winter" of oppression that many of these countries have experienced in the past.
The Arab Spring began in Tunisia, with the self-immolation of a vegetable seller named Mohammed Bouazizi. This kicked off a wave of protests that ended with the ouster of Tunisian president Zine el Abidine Ben Ali. Similar protests (and self-immolations) soon followed around the Middle East/North Africa region. More information can be found in pretty much any newspaper