AKB48 will release their 21st Single (title unknown as of yet) on 25 May 2011. The Single will come in a total of 3 different versions, including the Theater version. Included inside the single are voting tickets for, you guessed it, the AKB48 3rd Senbatsu General Election. Not much else is known of the Single as of yet, and further details will be announced later. They will donate a part of earnings of this Single for the earthquake relief.
The AKB48 3rd Senbatsu General Election will be for the still unannounced 22nd Single. The Senbatsu Elections is an annual event started in 2009 with Namida Surprise, where fans get to vote for who they would want to get into the Senbatsu (for that Single alone). It is essentially a popularity contest, or as I would say, "Whose fans got the most money". It has not been announced yet but most likely since SKE48 was included in the last two elections, both SKE48 and NMB48 will be part of the Senbatsu Election too. Voting ends on 8 June.
The previous winners of the Senbatsu Elections are Maeda Atsuko in 2009, and Oshima Yuko in 2010.
Oh and this is NOT an April Fool's joke.