There's a lotta stuff going on in one place here. Breathe deeply; take it slow.
Your email accounts are listed here; this is also where you set up new ones. See Free Email Accounts for details.
Here, you set up your email account information, specify how often you want the iPhone to check for new messages, change the font size for email, and more.
Show. Using this option, you can limit how much mail the Mail program shows you, from the most recent 25 messages to the most recent 200. This feature doesn't limit you from getting and seeing all your mail—you can always tap Download More in the Mail program—but it may help to prevent the sinking feeling of Email Overload.
Preview. It's cool that the iPhone shows you the first few lines of text in every message. Here, you can specify how many lines of text appear. More means you can skim your inbound mail without having to open many of them; less means more messages fit without scrolling.
Minimum Font Size. Anyone with fading vision—those of us over 40 know who we are—will appreciate this option. It lets you scale the type size of your email from Small to Giant.
Show To/Cc Label. If you turn this option on, a tiny, light gray logo appears next to many of the messages in your In box. The
logo indicates that this message was addressed directly to you; the 
logo means that you were merely "copied" on a message that was primarily intended for someone else.If there's no logo at all, then the message is in some other category. Maybe it came from a mailing list, or it's an email blast (a BCC), or the message is from you, or it's a bounced email message.
Ask before deleting. Ordinarily, you can delete an email message fast and easily (Filing or Deleting One Message). If you'd prefer to see an "Are you sure?" confirmation box before the message disappears forever, turn this option on.
Always Bcc Myself. If this option is on, then you'll get a secret copy of any message you send; see Reading Mail for the rationale.
Signature. A signature, of course, is a bit of text that gets stamped at the bottom of your outgoing email messages. Here's where you can change yours; see Signatures for details.
Default account. Your iPhone can manage an unlimited number of email accounts. Here, tap the account you want to be your default—the one that's used when you create a new message from another program, like when you're sending a photo or tapping an email link in Safari.
Now that Contacts is a first-class citizen with an icon of its own on the Home screen, it also gets its own little set of options in Settings.
Sort Order, Display Order. The question is: How do you want the names in your Contacts list sorted—by first name or last name?
Note that you can have them sorted one way, but displayed another way. This table shows all four combinations of settings:
Display "Last, First"
Display "First, Last"
Sort order "First, Last"
O'Furniture, Patty Minella, Sal Peace, Warren
Patty O'Furniture Sal Minella Warren Peace
Sort order "Last, First"
Minella, Sal O'Furniture, Patty Peace, Warren
Sal Minella Patty O'Furniture Warren Peace
As you can see, not all of these combinations make sense.
Import SIM Contacts. If you come to the iPhone from another, lesser GSM phone, your phone book may be stored on its little SIM card (SIM Card Slot) instead of in the phone itself. In that case, you don't have to retype all of those names and numbers to bring them into your iPhone. This button can do the job for you. (The results may not be pretty. For example, some phones store all address-book data in CAPITAL LETTERS.)
Your iPhone's calendar can be updated by remote control, wirelessly, through the air, either by your company (via Exchange, Chapter 15) or by somebody at home using your computer (via MobileMe, Chapter 14).
New Invitation Alerts. Part of that wireless joy is receiving invitations to meetings (Exchange + MobileMe), which coworkers can shoot to you from Outlook—wirelessly, when you're thousands of miles apart. Very cool.
Unless, that is, you're getting a lot of these invitations, and it's beginning to drive you a little nuts. In that case, turn New Invitation Alerts off.
Sync. The new wireless sync feature also accounts for the Sync option here. If you're like most people, you refer to your calendar more often to see what events are coming up than what you've already lived through. Ordinarily, therefore, the iPhone saves you some syncing time and storage space by updating only relatively recent events on your iPhone calendar. It doesn't bother with events that are older than 2 weeks old, or 6 months old, or whatever you choose here. (Or you can turn on All Events if you want your entire life, past and future, synced each time—storage and wait time be damned.)
Time Zone Support. Now, here's a mind-teaser for you world travelers. If an important event is scheduled for 6:30 p.m. New York time, and you're in California, how should that event appear on your calendar? Should it appear as 3:30 p.m. (that is, your local time)? Or should it remain stuck at 6:30 (East Coast time)?
It's not an idle question, because it also affects reminders and alarms.
Out of the box, Time Zone Support is turned on. That is, the iPhone automatically translates all your appointments into the local time. If you scheduled a reminder to record a TV movie at 8:00 p.m. New York time, and you're in California, the reminder will pop up at 5:00 p.m. local time.
Note
This presumes, of course, that the iPhone knows where you are. Even though the iPhone always knows what the local time is when you travel across time zones, it can't actually determine which time zone you're in. You have to tell it each time you change time zones—by tapping Time Zone here on this screen.
If you turn Time Zone Support off, then everything stays on the calendar just the way you entered it.
Default Calendar. This option lets you answer the question: "When I add a new appointment to my calendar on the iPhone, which calendar (category) should it belong to?" You can choose Home, Work, Kids, or whatever category you use most often.