Text Layers allow you to add any kind of annotation on your layout. To add a Text on a Text layer, simply double click on the layout, it will pop-up a window where you can edit your text, and after clicking on the Ok button, the text will be added at the place you double clicked. Later on, if you want to modify a text, double-click on it again, it will bring again the window to edit the text.
You can type your text on several lines in the edit box 1, and for each text, you have a full control over the Font, the Size, the Style (bold, italic, etc...) 2, the color 3 and the alignment (left, center, right) 4. However, you cannot adjust the transparency of each Text independently but you can change the transparency of the Text Layer by double clicking on the text layer tab. So if you want to have different transparencies for different texts, just use different Text Layers. Default Text properties can be set in the Appearance Preferences.
Texts can be rotated by any angle, their rendering order can be adjusted ("Send to Back" and "Bring to Front"), and of course they can be moved but they are not affected by the grid snapping. They also can be copy/pasted and grouped just like Parts.