When you reply all, it goes out to everyone on the To line and the CC line. Reply only goes to the sender. When we send out a mail to many people, you might see a sentence that says 'little 'r' me'. This means to use the small R instead of the Big R. That basically means to reply only to me instead of replying to everyone on the mail thread.
A mail is also referred to as a thread. That is because in the office, a mail can go on back and forth among many people so it becomes a chain of mail that started from one email. I'll use these terminologies in the next several example sentences.
تعلم تحدث الانجليزية أسرعبواسطة تحميل برنامج TalkEnglish الاوفلين. واحط نفسك باكثر من 8000 ملف صوتي واكثر من 800 صفحة من الدروس!. يمكنك الدراسة دون الاتصال بالانترنت والاستماع الي الملفات الصوتية بواسطة مشغل MP3 في اي وقت. اذهب الي حمل الانجليزية وحمل اليوم!