Below is a short description about each button.

Bold text style.
Italic text style.
Underline text style.
Strikethrough text style.
Align left.
Align center.
Align right.
Align full.
Unordered list/bullet list.
Ordered list/numbered list
Outdent/decrease indentation.
Indent/incread indentation.
Undo the last operation.
Redo the last operation.
Insert a new link, read more about this function in the Insert link section.
Unlinks the current selection/removes all selected links.
Insert a new image, read more about this function in the Insert image section.
Cleanup code/Removes unwanted formating. This function is useful when you copy contents from for example a office product.
Shows this help window.