The wysiwyg (what you see is what you get) editor can be used in the admin area when adding/editing an HTML module or when editing a page in-situ.

The toggle toolbar mode link hides and opens the second row buttons

Top row buttons: (from left to right)

Insert/edit file
B - Bold
I - Italic
U - Underline

Left align
Center
Right align
Justify

-- Format -- - A list of available formats for text. eg. paragraph, heading1, heading2

Unordered list
Ordered list

Outdent
Indent

Undo
Redo

Link (add a link to highlighted text or by clicking on an image)
Unlink (unlinks a HTML link)
Insert/edit anchor
Cleanup messy code
Spell check

Second row buttons: (from left to right)

Delete table (only available when a table is created)
Insert table
Table row properties (only available when a table is created)
Table cell properties (only available when a table is created)
Insert row before (only available when a table is created)
Insert row after (only available when a table is created)
Delete row (only available when a table is created)
Insert column before (only available when a table is created)
Insert column after (only available when a table is created)
Remove column (only available when a table is created)
Split merged table cells (only available if there are merged cells in a table)
Merge table cells (only available when a table is created)
Edit html source