Mail app: general layout, and how to read, send, and manage messages
There is a very accessible mail app built into iOS. However, there are a few things that it took me a bit to figure out, so I figured I would share my findings for anyone who has the same questions. Please note that I have only set up a GMail account, so I cannot say exactly how other account types will work, but the steps should be similar. Also, this is on an iPod Touch 4th generation running iOS 4.2.1 (latest as of Dec 30, 2010).
When you have created your account, you will be in your inbox. From here, double tap any message to open it, or hit the back button (or two-finger scrub) to go back to a list of all your folders (in GMail, your labels correspond to folders).
In any folder, the back button is, as usual, in the top left corner of the screen. In the middle is a search field, which will let you search through all messages in that folder. In the top right corner is an edit button, which I will return to in a moment.
Below these three controls is the list of all messages; the usual three-finger swipes will move you through this list by page, and flicking right or left will move you forward or back a message. The app seems to display an email's information in a certain order which I do not think can be changed. To open a message, just double tap or split tap like you would for any other control. Below the messages list are some buttons and text arranged along the bottom of the screen:
*refresh: updates the current folder.
*updated: a text heading telling you when the folder was last updated.
*[date] this is the last time the folder was updated.
*compose: lets you write a new message.
If you have selected messages (see below), the above buttons will be replaced by two buttons: archive and move, with the number of messages selected spoken after each. Again, these may show up differently if an account type other than GMail is being used.
If I select 8 messages, for example, then touch the bottom left of the screen, I will hear "archive. 8. button." To the right of that I will hear "move. 8. button."
To select multiple messages, double tap the edit button in the top right corner of the screen. Doing this will change the behavior of double tapping a message: when edit is on, double tapping a message will select it, not open it. Once you have selected all the messages you wish to delete, for example, just find the "move" button and double tap it, then select the trash folder (again, this may be GMail-specific). If you decide you do not want to do anything with the selected messages, just go back to the top right of the screen; you will find a cancel button where the edit button was. Double tap this and edit mode will be turned off, returning the mail app to its default of opening a message when you double tap. Canceling, of course, will also deselect all of your selections.
When you open a message, you will see a back button in the top left with the name of the parent folder, followed by a heading telling you which message this is (such as 1 out of 42 or 5 out of 16), followed by "previous message" and "next message" buttons. Below that is the sender (whose address is a button you can click to bring up options for the sender), the subject, and the date of the message. Oddly, these seem only to appear when I flick right or left, not when I move my finger around the screen, but they can be accessed. These are followed by the body. Below the message's text is a row of buttons. For a GMail account, they are:
*refresh
*move
*archive
*reply (prompts to forward, reply, or reply to all)
If you double tap "compose", you will be placed in a screen where you can compose a new message. The top left has a cancel button in place of a back button (note that scrubbing will not activate the cancel button), and to the right is a "new message" heading, then a send button in the top right. If you cancel, you are prompted to save or delete the draft (in other words, save or not save a draft of the message), or cancel the cancel and go back to the message.
Next comes the "to" field, which is focused on by default and waiting for you to type something. Once you start typing, suggestions from your contacts will appear above the keyboard. Double tap a name to add it. Once you add a name, all suggestions disappear, text in the field is cleared, and the most recent name appears above the "to" field. Focus on the name and flick left and right to hear the other names you have selected already. I have not yet found a way to remove a name from this list, except for canceling and starting over. There is also a "add from contacts" button on the right, which will bring up the contacts app so you can choose from your contacts. Using GMail at least, and assuming you have an internet connection, contacts seem to be gotten from my GMail contacts, not my iPod's contacts (no, they are not yet synchronized), so do not be surprised if you get contacts showing up that you did not think were on your iTouch device.
The cc/bcc button, when double tapped, will bring up "cc" and "bcc" fields, which both work exactly the same as the "to" field in terms of typing, suggestions, and so on.
The subject field is just a normal edit field, as is the body (though the body field holds your auto signature, configurable from settings, and is multi-line). Again, the send button is in the top right of the screen. If the message is sent, and if you have the sound for mail turned on, you will hear the sound of the message leaving. If there is an error, you will be told to make sure your settings are correct and try again. If you manually enter an email address and it does not appear valid, iOS will ask a few times if you are sure you want to continue.
Notes:
*As mentioned above, to forward a message, go to the reply button; there is no dedicated forward button, at least not on the iPod Touch.
*If a link (or smart link, such as a date) is in a message, you can double tap and hold to bring up options, just like in Safari.
*There is currently no way to attach files to an email. Personally, I would use a Dropbox (http://www.dropbox.com) account and just embed the link to a file if I really had to send it to someone. Also, forwarding emails with attachments works, so you could send yourself a message with an attachment and then forward it if necessary.
*To save a draft, double tap cancel, then select the "save draft" button; there is no dedicated "save draft" button from the compose screen.
If I missed anything or got anything wrong, let me know in the comments!

Comments
#1 Sending Attachments in Mail
While there is no general way to send file attachments initiated from the Mail app, this is because the originating app has to generate or have access to the content to be attached and routed to Mail. Thus, you can select images in the "Photos" app, or generate an audio memo in the "Voice Memos" app and then double tap an "Actions" button and select an option to email the photos or voice memo as an attachment. You can send file attachments via Email using a third party app like Filer (formerly Downloader) to attach files you have downloaded using the app, including Dropbox files.