By default, Gmail groups all replies with their original message, creating a single conversation or thread. Replies to emails (and replies to those replies) are displayed in one place, in order, as a single group.
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. This is the "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.
Now, you have the option to change this setting so that replies aren't threaded into conversations, but appear as individual messages in your inbox.
To "unthread" your e-mail conversations, go to the General tab of your Gmail Settings, and select Conversation view off.