It's 'not just for fun'
The way I see it, it wouldn't be Battlefield without a beta. In recent years, I've transitioned into becoming more of a casual player of the series and, like with Battlefield 3, my experience with the new game, at least early on, might be entirely based on what's in the BF4 beta.
It'll be here before we know it. The beta kicks off for everyone on October 4, 2013 (Xbox 360, PC, PlayStation 3) but those who pre-order the Digital Deluxe edition of Battlefield 4 on PC, are a BF3 Premium member, or are a registered owner of Medal of Honor Warfighter's Limited or Digital Deluxe edition will gain entry on October 1.
As for modes and maps, it's Conquest and Domination on the Siege of Shanghai. Here's a quick runthrough of the content from DICE.