Joomla! comes with an editor called 'tinyMCE'. We like an enhanced version called 'Joomla Content Editor' or JCE.
There are also others that can be installed. In the JCE editor, only icons that can be used are active. For example there are a number of icons for editing tables that are not active unless you have a table selected. To create a web link you must first have selected some text or an image to apply the link to. When you hover over icons you see a short description of the function. The very first icon brings up a help page for the editor.
Notes on a few special functions:
Styles: the styles dropdown should include any styles defined by your template in the file templates/your_template/css/template.css
Your web developer should place any special styles for your site in that file.
Insert/Edit Image. Place the cursor where you want the inmage to appear and click this icon.
Insert/Edit Link. Highlight some text or an image and click this icon.
[Show/hide]: one click hides the editor and displays the raw HTML which can be edited.
second click shows the editor and hides the raw html.
Advanced code editor. Opens an html editor with code highlighting. Better than editing the raw source.
Paste from Word. If you copy text from Microsoft Word, use this to paste from the clipboard. MS Word surrounds the text with its own style information and it is best to strip that out as it will not match your site styles.
Toggle Full Screen. Nice to have more space to work with. Click a second time to come back.