Farm solutions are hosted in the IIS worker process (W3WP.exe).If you run any code in farm solution the whole farm will got affected. If you deploy any feature or retract any feature the whole application pool got recycled.Since they are scoped as farm level, they have full trust access to all the resources.When the Sand boxed Solution property is set to False, selecting Build\Deploy will deploy the solution to the Farm Solution Gallery. Sand boxed solutions are hosted in the SharePoint user code solution worker process (SPUCWorkerProcess.exe).The process runs under a CAS policy that restricts programmatic access to any resource outside the sandbox. So it never restart the IIS application pool.If you run any code it will affect only the site collection of the solution.When the Sandboxed Solution property is set to True, selecting Build\Deploy Solution deploys the solution to the site collection Solution Gallery.Sandboxed solution is scoped to the site collection.

• Application pages.
• Visual web parts.
• Code-based workflows.
• Connect to resources that are not located on the local farm.
• Access a database.
• Call unmanaged code.
• Access resources in a different site collection.
• Change the threading model