magento module files and design files location

Modeule installation xml file, required.
app/etc/modules/PackageName_ModuleName.xml

Module block files, required for custom block to be used in template files. In the layout xml specify the block class type and template file location.
app/code/local/PackageName/ModuleName/Block/SomeBlock.php
app/code/local/PackageName/ModuleName/Block/Adminhtml/SomeAdminBlock.php

Module controllers, required for custom controller.
app/code/local/PackageName/ModuleName/controllers/IndexController.php
app/code/local/PackageName/ModuleName/controllers/SomeController.php
app/code/local/PackageName/ModuleName/controllers/Adminhtml/SomeAdminController.php

Module config xml files, required.
app/code/local/PackageName/ModuleName/etc/adminhtml.xml
app/code/local/PackageName/ModuleName/etc/cache.xml
app/code/local/PackageName/ModuleName/etc/config.xml
app/code/local/PackageName/ModuleName/etc/system.xml

Module helpers, required for custom helper functions.
app/code/local/PackageName/ModuleName/Helper/Data.php
app/code/local/PackageName/ModuleName/Helper/OtherHelper.php

Module model classes, required for custom models such as custom tables in magento database.
app/code/local/PackageName/ModuleName/Model/Obserber.php
app/code/local/PackageName/ModuleName/Model/SomeModel1.php
app/code/local/PackageName/ModuleName/Model/SomeModel2.php
app/code/local/PackageName/ModuleName/Model/Resource/Setup.php
app/code/local/PackageName/ModuleName/Model/Resource/SomeModel1.php
app/code/local/PackageName/ModuleName/Model/Resource/SomeModel2.php
app/code/local/PackageName/ModuleName/Model/Resource/SomeModel1/Collection.php
app/code/local/PackageName/ModuleName/Model/Resource/SomeModel2/Collection.php

Module sql installation script, for create or update custom mysql table recommended but not required because you can create or update the custom mysql table directly in the magento database.
app/code/local/PackageName/ModuleName/sql/modulename_setup/mysql4-install-0.1.0.php
app/code/local/PackageName/ModuleName/sql/modulename_setup/mysql4-install-0.1.0-0.1.1.php

Design layout and template files, required for custom frontend layout and rendering.
app/design/adminhtml/layout/default/layout/some_layout.xml
app/design/adminhtml/layout/default/layout/some_template.phtml
app/design/frontend/layout/packagename/themename/layout/some_layout.xml
app/design/frontend/layout/packagename/themename/template/some_template.phtml

Magento hello world module