I can't speak for people in general, but I'm an INFJ and I generally communicate affection through actions. When I care about someone, I like to do little things for that person. For instance, my good friend and roommate loves tea in the morning. Usually I wake up before her, so I try to have tea waiting for her when she wakes. It's something small, but it makes her happy.
I'm also very comfortable telling people verbally how I feel about them. However, I find this makes some people uncomfortable, so I try to temper my words a bit depending on the person I'm addressing.