This tag is for IDEs that are basically web apps. Any online tool that offers more features than just simple text editing should use this tag. However, consider using the product-specific tag instead if that tag exists (E.g. [sap-web-ide] instead of [web-ide]).
A web IDE is a development tool that is used through a browser. It allows a developer to write code in an environment that is very similar to a desktop development environment. These tools typically include:
- syntax highlighting
- error checking
- version control support
- plugins to various cloud-based services
- a virtual machine to run compiled code against
More information and examples can be found on the wikipedia entry for web IDE
