The iPhone Email application lets you receive emails from standard email accounts using POP (Post Office Protocol), IMAP or Microsoft Exchange server. To set up an email account on your iPhone, see the article on setting up an iPhone email account.
Once you have an email account or two set up on your iPhone, simply tap the Mail application and you will get either the Account screen, the main screen for the last account you were viewing, or the inbox for the last account you were viewing. It depends on where you last were when using Mail.

When you enter Mail, if you are looking at the inbox you will also automatically start retrieving new mail from the server unless you have that turned off in your Mail settings. If you are using a Yahoo email account, new mail messages will be pushed to your iPhone without you even needing to access the Mail application.
If you are viewing the inbox, you can simply tap on an email to view the email. You can also swipe left to right to bring up a delete button. Or, to delete email you can tap the Edit button at the upper right and you will get red buttons to the left of each email. Tap those to bring up a Delete button.
Email you haven’t read yet will have a blue dot next to it.
At the bottom of the inbox screen you’ll see a Updated time and date. This tells you when Mail last had the chance to get all your messages from the server.
When you view an email, you will see the From and To fields at the top of the screen, and the message below. You will also see any additional recipients, such as people that are CC’ed. You can hide the recipients by tapping the word “Hide” at the upper right.

At the top of the screen are buttons that take you back to the inbox, and arrows that let you page through all of the messages in your inbox.
While looking at a message, you will also see any attachments. The iPhone can actually open and display a lot of different attachment types, such as PDF (Adobe Acrobat), Word, Excel and many image file types. It can also play many sound and video file attachment.
Even if an attachment type is supported, such as a .wav sound file, the iPhone may not be able to play it back as there are many variations inside a single file type. PDF files may not show some of the more advanced features of the PDF format, for instance.
At the bottom of the message, you can see buttons that perform various functions. The first button, a circle/arrow, will retrieve more email from the server. The second button, a folder with a down arrow, will allow you to move the message out of the inbox to another folder. The trash can lets you delete the message. The arrow pointing left will let you Reply, Reply to all of the recipients, or Forward the message. The last button, a pencil with a box, lets you start composing a new message.
Moving the message to another folder won’t work unless the account on the server is set up with multiple folders. Most POP accounts won’t have anything except Inbox, Sent and Trash. But Web-based email like Yahoo and Gmail give you the ability to create other folders.

When you start composing a new message, you would usually first enter the recipients email address, or use the + button to add one from your Contacts. You can skip the Cc line unless you want to send a copy of the email to someone else as well. Then you can enter a subject.
Finally, enter a message. Your default signature is automatically included in the message.