You can view your email messages as either Messages or as Conversations.
To change how messages are grouped, go to the Preferences>Mail tab and change Group mail by.
You can also view messages in a three-pane view, with the Reading Pane on the Left.
Messages
Email grouped as messages are displayed with the most recent message first in your email folders. Unread messages are shown in bold. When you review the message line, you can see who the message is from, whether there is an attachment, the subject of the message, in which folder the message is saved, the size of the message, and when it was received. If messages have a flag enabled or tags, these are also shown.
Conversations
If you group mail by conversations, your messages are grouped by subject. A conversation is a group of messages that have the same subject. If the subject is changed, the message is not included in the conversation.
The subject displays only once in the Conversation pane. The number of messages in the conversation is displayed in parentheses after the subject. When new email comes in, the conversation is moved to the top of your email messages. The sender list displays the names of the last eight participants in the conversation. Conversations containing unread messages are shown in bold.
Conversation View
Double-click the conversation to display the messages within the conversation. Click on a message to display its contents in the Reading Pane below.
In the message toolbar, you can click the 
icon to display the message in a new window.
All messages related to the conversation are displayed, even if the messages are stored in other folders. The Folder column shows the folder where the message is stored. Conversations do not continue indefinitely. If the last message in the conversation is 30 days old or older, messages sent with the same subject begin a new conversation.
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