Gmail groups all replies with their
original message, creating a single conversation or thread. In other
email systems, responses appear as separate messages in your inbox,
forcing you to wade through all your mail to follow the conversation. In
Gmail, replies to emails (and replies to those replies) are displayed
in one place, in order, making it easier to understand the context of a
message -- or to follow the conversation.
When
you open one message in a conversation, all of your related messages
will be stacked neatly on top of each other, like a deck of cards. We
call this Conversation View. In Conversation View, each new message is
stacked on top of the ones that arrived before it, so that the newest
message is always the one you see first.
To see all the messages in a conversation, just click the "Expand all" button. 
A
conversation will break off into a new thread if the subject line of
the conversation is changed, or if the conversation reaches over 100
messages.
If you'd like,
you can change this setting so that replies aren't threaded into
conversations, but appear as individual messages in your inbox. To do
so, go to the General tab of your Gmail Settings, and select the radio
button next to 'Conversation view off'.
