Whenever you click the pencil icon or the NEW POST button, you will open the post editor which can be either in the HTML mode or the Compose mode (WYSIWYG or What You See Is What You Get mode) selectable by clicking the relevant tabs at the top left of the post editor.
The number of tool icons available in the tool bar of the 2 different modes are different, less in the case of HTML mode and more with the COMPOSE mode:
While the HTML mode requires one to have some knowledge of HTML coding, it can give you more control over your blog. Furthermore, even a little knowledge of HTML can go a long way towards improving your posts. I very much prefer to use the HTML mode.