Here are some of the typical template files that you might want to customize. Assume "example" is your custom template folder.

The template files for the home page:
includes/templates/example/common/html_header.php (for the doctype and head)
includes/templates/example/common/tpl_header.php
includes/templates/example/common/tpl_main_page.php
includes/templates/example/common/tpl_footer.php
includes/templates/example/templates/tpl_index_default.php (welcome text, greetings, and centerboxes)
includes/languages/english/example/header.php (text definitions)
includes/languages/english/thml_includes/example/define_main_page.php (some main page text)

Template Files for EZPages
includes/template/example/templats/tpl_page_default.php

Page Not Found
You can view the "404 page" by viewing a bogus URL such as http://www.example.com/index.php?main_page=xyz

Text is in includes/languages/english/html_includes/example/define_page_not_found.php

If you want, can make the 404 page do an automatic Javascript redirect to your home page by adding ...

<script>
window.location = "http://www.example.com/"
</script>

Other Predefined Page
ZenCart has some predefined page which you may also want to customize. They can be view by these URLs...

http://www.example.com/index.php?main_page=index (home page)
http://www.example.com/index.php?main_page=index&cPath=1 (category page)
http://www.example.com/index.php?main_page=advanced_search (advanced search)
http://www.example.com/index.php?main_page=specials (specials)
http://www.example.com/index.php?main_page=reviews (reviews)
http://www.example.com/index.php?main_page=shippinginfo (shipping and returns)
http://www.example.com/index.php?main_page=privacy (Privacy Policy)
http://www.example.com/index.php?main_page=conditions (conditions of use)
http://www.example.com/index.php?main_page=contact_us (contact page)

Their text content can often be found in the folder ...
/languages/english/html_includes/example

Database Not Found Page
/nddbc.html

Down for Maintenance Page
Page: http://www.example.com/index.php?main_page=down_for_maintenance
Template: /includes/templates/template_default/templates/tpl_down_for_maintenance_default.php
Text: includes/languages/english/down_for_maintenance.php
Image: ncludes/templates/template_default/images/down_for_maintenance.gif
Image filename definition: includes/languages/english/other_images_names.php

Template for Product Detail Page
http://example.com/index.php?main_page=product_info&cPath=1&products_id=8
Template: includes/tempalte/template_default/templates/tpl_product_info_display.php

Product List Page
Page: http://www.example.com/index.php?main_page=index&cPath=1
Template: includes/templates/template_default/templates/tpl_modules_product_listing.php

Shopping Cart Page
Text: includes/languages/english/shopping_cart.php
Page: http://www.example.com/index.php?main_page=shopping_cart
Template: includes/templates/template_default/templates/tpl_shopping_cart_default.php

Shipping Estimator Page
Page: http://www.example.com/index.php?main_page=popup_shipping_estimator
Text: includes/languages/english.php
Template: includes/templates/template_default/templates/tpl_modules_shipping_estimator.php

Help Page
Page: http://www.example.com/index.php?main_page=info_shopping_cart
Text: includes/languages/english/info_shopping_cart.php
Template: includes/templates/template_default/info_shopping_cart/tpl_main_page.php

Button Images and Icon
includes/templates/template_default/images
includes/templates/template_default/buttons

Checkout Page1 - Shipping
Page: http://www.example.com/index.php?main_page=checkout_shipping
Templates: includes/templates/template_default/tpl_checkout_shipping_default.php

Checkout Page2 - Payment
Page: http://www.example.com/index.php?main_page=checkout_payment
Template: includes/templates/template_default/templates/tpl_checkout_payment_default.php

Checkout Page 3 - Confirmation
Page: http://www.example.com/index.php?main_page=checkout_confirmation
Template: includes/templates/template_default/templates/tpl_checkout_confirmation_default.php

Checkout Success Page
Page: http://www.example.com/index.php?main_page=checkout_success
Text: includes/languages/english/checkout_success.php
Template: includes/templates/template_default/templates/tpl_checkout_success_default.php

Checkout Change Ship Address
Page: http://www.example.com/index.php?main_page=checkout_shipping_address
Text: includes/languages/english/checkout_shipping_address.php
Template: includes/templates/template_default/tpl_checkout_shipping_address_default.php

Checkout Change Billing Address
Page: http://www.example.com/index.php?main_page=checkout_payment_address
Template: includes/templates/template_default/t

My Accounts Page
Page: http://www.example.com/index.php?main_page=account
Text: includes/languages/english/account.php
Template: includes/templates/template_default/templates/tpl_account_default.php

Account History Info
Page: http://www.example.com/index.php?main_page=account_history_info&order_id=1
Template: includes/templates/template_default/templates/tpl_account_history_info_default.php

Account Created Page
Page: http://www.example.com/index.php?main_page=create_account_success
Template: includes/templates/template_default/templates/tpl_create_account_success_default.php