Whatsapp one of the best and popular chating app in android windows ios it is also availabe in pc also .i am using it since 3 years and I have a awesome experiance becasue it is free of charge and its only requier data connection like 2g 3g 4g and wifi .nowadays it gives you calling option also and feature of whatsapp is you can send imegas videos mp3s gifs documents like pdf docs etc the best part of it is chating you can chat with your friend wherever they are in the world you can simply chat with them at free of cost.for one no and one phone you can use only one whatsapp account .if you chenge you your mobile simply you can switch same account with your same number