Web development environment often termed IDE is a program that often will let you code, see results, execute scripts, works with a local server and more. Dreamweaver and Netbeans are examples of an IDE. Text editor is good for writing up straight code but not possible to create a complete site that can be thoroughly tested out as a whole. Notepad ++ and Sublime text are a couple of examples.
Using an IDE for developing is a lot more comfortable, but not really necessary to write code. For simple things like writing a HTML page and some css a text editor is enough.