SharePoint solutions are packaged as .wsp files that can contain features, site definitions, Web Parts, and assemblies. There are two kinds of solutions: farm and sandboxed. Farm solutions are deployed on front-end Web servers by a farm administrator, have full access to the server object model, and are not subject to any usage limits.
In SharePoint Online you cant access the farm.
By comparison, sandboxed solutions can be deployed by a site collection administrator — or by a user who has the Full Control permission level at the root of the site collection — into the solution gallery for a site collection e.g. on SharePoint Online in Office 365.
Sandboxed solutions have limited access to the server object model and run in a security restricted context that provides isolation and monitoring of the sandboxed solution's code. It is not always easy for a developer to remember which restrictions apply to sandboxed solutions in SharePoint Online.
IntelliSense to the Rescue
Most Visual Studio developers find IntelliSense tremendously useful for discovering API features, syntax, and usage. But it’s less than useful if it shows APIs your project can’t use. In Visual Studio 11 IntelliSense has been improved when developing remote solutions to display only APIs that are applicable to sandboxed solutions. Also, red squiggles and compile errors appear if your code contains any references to APIs that won’t work in sandboxed solutions.
This is only one of the improvements for SharePoint Online developers in Visual Studio 11 - read more in this blog post from the SharePoint Developer Team.