Today is the election of the Senators for the upper house of the Thai parliament, the Senate (วุฒิสภา). The senate has mostly control functions, so it has to approve the bills (laws) issued from the House of Representatives, approve appointments of some independent organizations, and can also start impeachment procedures. The senators are elected for 6 year term.
In the past the senate consisted only of appointed senators, and it was the 1997 constitution which introduced the election of senators. The first senate election took place in 2000. The second one in 2006 was at the height of the political crisis, which culminated with the military coup in September. The military dissolved the senate even before it had its constitutive convention. The 2007 constitution partially changed back to an appointed senate, now 74 of the 150 senators are appointed, and another 76 is elected. As one can already guess from the number 76, the constituencies for the senatorial elections are simply the 76 provinces, every province regardless of its population is eligible of sending one senator.
In Thailand voting is compulsory, however the only punishment for voters who fail to show up for the ballot is the temporary loss of passive voting rights - they cannot become subdistrict or village headman, senator or member of the house of representatives, as well as losing the right to file any objection to an election. One regains there rights when voting at any election, regardless whether it is a local or a country-wide. Though the Public Relations Department (PRD) expects a voter turnout of 70%, I guess the final number will be much lower, given the fact that most voters don't know their candidates - the senatorial candidates are very limited in their election campaign.
I have found a nice article "Electoral Politics in Thailand" by Orathai Kokpol, which gives a nice overview on the history of elections in Thailand. It was published in 2002 as part of the book "Electoral politics in Southeast & East Asia", which is completely available online. Also, the New Mandala blog just presented two papers by Michael H. Nelson on the recent elections.