This article from UsingJoomla.com is introducing you to a very useful local editor.
We use the term local editor because this is installed on your computer or laptop.
As you become more familiar and comfortable working with your Joomla! powered website you may want to create custom cascading style sheets (css) - we all know what it is like to want to change the colour, or appearance of something - such as place a border around a module which makes that module stand out from other modules.
There can be an issue with using something like Microsoft Word (sorry Microsoft !) in that you will preferably want to use a "clean" editor which does not use unseen (hidden) formatting code.
We here at UsingJoomla.com recommend using 
Notepad++, sometimes perhaps referred to as Notepad plus plus.
With Notepad++ a very wide variety of files can be opened for editing, as well as creating new files.
Files can be "saved as" into many different formats, which you will surely encounter in your journey with Joomla!, such as
- .js (Javascript file)
- .txt (normal text file)
- .php (Hypertext PreProcessor file)
- .css (Cascading Style Sheet file)
- .scss (Sass Cascading Style Sheet file) - though an additional plugin needs to be installed for this
- .html (HyperText Markup Language file)
Although very useful for editing existing files, and creating new files a word of warning from UsingJoomla.com follows:
WARNING
Joomla!® contains many, many, many core files.
Alter Joomla files with caution.
Why?
- Whenever there is an update to Joomla some, or all, core files get overwritten with newer versions - if you have changed a core file the update will remove your customisations.
Besides the above:
- Changing one (or more) core Joomla file(s) can cause other features of Joomla to stop functioning, or function incorrectly.
- Many extension developers will not provide support on a Joomla site where core Joomla files have been changed.
- Changing one (or more) core Joomla file(s) can break your site - we recommend that you DO NOT change ANY core Joomla file(s).
YOU HAVE BEEN WARNED !
So - if this is the case, why are UsingJoomla.com writing about an editor to create and edit files ?
Good question, followed by . . .
The good news
If you wish to make css customisations to your template, your template provider should be able to offer advice on how to accomplish this with a custom css file in your /templates/mytemplate-name folder.
Such advice will usually come in your template provider's documentation, FAQs, support forum, or similar feature on their own website.
Such custom css files do not normally get over-written by Joomla updates and indeed is seen as the correct way of carrying out custom css.
Why do developers not provide support when you have changed core files ?
Well, see it like this . . .
. . . someone gets a car from you. They change things. Then something does not work.
Are you going to offer support ?
