"Tell" is typically used when you're saying something specific and short (or just specific). It implies that the person you're speaking to isn't actually responding. "Talk" is typically used when the person you're speaking to is able to also speak, or if you're talking AT them about something long and non-descript. Examples: Tell him that I won't be there until 3 PM. I was talking about this hilarious tv show with my mother yesterday. I am telling you to stop now! I am talking to my friend on the phone, so give me a moment. He told me that he was going to buy a dog. She talked at me for hours on end, and I can barely remember a word she said!

Good question! Talking is a two-way conversation between person A and person B. "They are talking" = "They are having a conversation" Tell is usually a command to speak. "Tell your dog to be quiet" is person A (me) talking to person B (you) to give a message to 'person C' = the dog.