A development environment (IDE) is essentially a text editor with additional features. These usually include auto-complete, the ability to create "projects," compling code, debugging code, and syncing code using version control. For most languages, the main difference is the ability to compile code. For languages which are not compiled (like PHp), the notable difference is whether you can create projects, and whether there are built in features like auto completion.
Most modern development environments, or software dedicated to creating complex projects, will also have an option to either view source code or live code. Depending on the project, you may be required to have a local database or system set up to actually view the live code. This is nice because you can view changes to your code without having to save the file and then open the file in a web browser - or save file, upload file, and then reload page / worry about caches.
As Cosmic said, autocomplete features are nice and being able to quickly debug your code through automated systems is awesome. Having to go through hundreds of lines of raw text in a text editor can be a pain if you don't know what you are looking for specifically.
