According to Roscosmos an ammonia leak in the US section of the International Space Station has necessitated the evacuation of that section of the station.  
From what I have read this diagram depicts where the no-go zone is. 


Image courtesy of the Russian Space Agency Roscosmos.  


To put this in perspective consider the following.  If you spill some strong amonia in your house you open a window.  In space you cannot do that.  So to fix this someone may need to put on a space suit, and work in tight quarters to patch the leak.  If the leak cannot be patched then the amonia will stay in that part of the station unless there is some way to suck all the air out, and replace it.  But to replace that atmosphere would not be a simple task.  

Here is what Nasa is tweeting about it

#NASA TV #ISS update in about 20 minutes. Crew is safe. No ammonia leak confirmed. Crew responded to coolant loop pressure increases.

— NASA (@NASA) January 14, 2015

The #Exp42 crew members are safe inside the Russian segment of the #ISS following an alarm in the U.S. segment at about 4 a.m. EST.

— NASA (@NASA) January 14, 2015

About 7:50 a.m. EST: A live #NASA TV update on the situation aboard #ISS. Watch http://t.co/6XtjOi1yJo

— NASA (@NASA) January 14, 2015

Hopefully this is just a bad sensor reading or something. Otherwise things could get very interesting aboard the ISS.

Update: #Exp42 crew informed by controllers that it's starting to look like a false indication, either a faulty sensor or computer relay.

— NASA (@NASA) January 14, 2015

UPDATE
False sensor reading. 

#Exp42 open US segment hatch&return 3:05pm ET. @AstroTerry & @AstroSamantha sample #ISS air, no ammonia indication. pic.twitter.com/W5MbHNCgqK

— Intl. Space Station (@Space_Station) January 14, 2015

The leak was determined to not be super serious if it was real. After sampling the air directly no indication of an Ammonia leak. In a situation where you can't open a window an overabundance of caution is called for.