As the name implies, it means parts of a web.
Web Parts are the building blocks for building web pages in SharePoint.
Web Parts are essentially ASP.NET web controls that inherit the web control base class in ASP.NET but have special abilities compared with ordinary ASP.NET web controls.
Sandbox Solution vs. Form Solution:
1. Sandboxed solutions are hosted in the SharePoint user code solution worker process SPUCWorkerProcess.exe)
Farm solutions are hosted in the IIS worker process (W3WP.exe)
2. Sandboxed solutions are specific site collection
Farm solutions are run at Form level
3. Mapped folders cannot be added to the project in Sand box solutions.
4. Sandbox solutions does not support
• Visual Web Parts
• Application Pages
• Custom Action Group
• Web Application-scoped Features.
• Farm-scoped features
• Workflow with code