Syntax highlighting is the coloring of certain syntaxes in a markup or programming languages. It is commonly used by source viewers, and editors used when programming, scripting, and coding generally.
Large source files can be quite overwhelming to developers, so Syntax highlighting has been integrated as a feature to improve readability of code. Many editors support multiple languages, some even allow their users to add new languages.
It also helps programmers spot errors in their code, in that it can highlight syntax errors.
Editors and other Tools
Many editors allow for customization of the color schemes used, but their highlighting may not always be entirely accurate.
If you are looking for editors, and other tools, then have a look in Web Designers Tools List.
The above list is far from complete, so if you know of other free tools with support for Syntax highlighting as a minimum feature, then don't hesitate to post them in the comments of the above article.