The difference between WordPress.com and WordPress.org can cause some confusion. Overall WordPress.org allows for complete customization of display/design/branding and also full control over functionality.
.com has limited number of themes to start with. every theme has some customizations like header/logo and bg color. paying for css upgrade gives another layer of possible customizations.
.org allows virtually any kind of look – complete branding and customization. 1000s of themes to start or build one from scratch.
- Basic posts, pages and categories. generally there is 1 sidebar and that’s it. So even if you find one theme with a look you like and another theme with useful sidebars that organize your featured content the way you like you can’t combine aspects of different themes.
- .com will put ads on your pages sometimes—and you are not allowed to put your own ads. (The ads only display to logged out users who are not regular visitors—there is an upgrade to control this feature.)
- Complex homepage options with sliders and other ways to highlight content
- multiple sidebars for different sections of a site
- multiple customizable menus
- sidebar items that are only active on certain pages/categories/logged in users (Widget Logic)
Most important are plugins that add all kinds of neat features. Here’s some examples of common functions I add for my clients via free plugins:
- flash based mp3 player and video players
- flash and non-flash savvy image gallery tool (nextgen gallery just upgraded itself to be savvy about iPad/iPhones that don’t support flash)
- more SEO options (search engine optimization)
- powerful form building for contacts and other info (gravity forms)
- google analytics (detailed stats on who visits your site and how they find out about it; integrates adwords ads.)
- many options for twitter/facebook/flickr integration (and when the next web service comes up a plugin will appear for it too)
- zemanta and other tools to help generate content/images
- custom fonts via typekit or cufon
- maintenance mode
- quotes collection (for a page or a sidebar)
- page navigation (for a view of posts the bottom of the page will say how many pages worth of older posts are available.)
-more control over what urls look like and other subtle things like that
-multiple users. fora big staff or for building a social network or private content.
-ability to sell ads
See also the WordPress.com explanation.