Looking at this piece of code, what does the colon do?

def partner_to(user)
    raise ArgumentError unless user
    user.id == recipient_id ? sender : recipient
 end

I got the code from this rails post:
http://groups.google.com/group/rubyonrails-talk/browse_thread/thread/7fdccddde9b97ffc/6112294580b9e0ce?lnk=gst&q=internal+mail&rnum=1#6112294580b9e0ce

I'm just asking because I've never seen the colon used before.