You can build various apps types on the VK platform: 

• mobile_apps Mobile apps. Access to API methods and simplified authorization with SDK for major mobile platforms: iOS, Android, Windows Phone.

• Sites and Widgets. External web services or widgets installed on sites. Sites may use either OAuth server authorization or Open API client authorization. Most of API methods are available.

• VK native apps. These are run inside the social network and are integrated using Flash or an IFrame element. Most API methods, as well as Payments API methods for user payments are available for these applications.

• Standalone/Mobile Applications. Client applications for various platforms (for desktop and mobile devices).
  • Mobile apps should use SDK for iOS, Android or Windows Phone for authorization and API requests.
  • Client apps for other platforms and server apps require authorization using OAuth protocol and may call all API methods.

In order to call most API methods, Standalone/Mobile applications and external sites require user (or application) authorization. The same applies to server-run code for applications of any type.

• OAuth authorization (client and server). Authorization mechanisms for Standalone/Mobile applications and external sites differ slightly and are not interchangeable.
• Open API authorization (client only). Only available for websites.

• API requests.
• Uploading files to VK server.

• Description of API methods — List of available API methods.
• Payments API — Payments API for applications.
• Ads API — Methods for advertising accounts.
• Data types — Descriptions of data types returned by API methods.

• Comments. A generic module for comments on third-party sites.
• Post. Allows to publish a post or comment from vk on your website.
• Communities. VK Community modules for third-party sites.
• Like. A button that allows users to 'like' objects and share links with friends.
• Recommendations. A module that lists most popular materials on the site according to Like widget data.
• Polls. A polling module.
• Authorization. A basic authorization module.
• Link posting. A button that allows users to share links on VK.
• Subscribe to an author. A button that allows users to subscribe to an author or community.

• API Change Log — Recent changes in the VK API.
• API status — Platform status.