This page is part of the Second Life User's Manual, describing how the Viewer software works. Please see the main User's Manual page as a starting point.
The "Appearance" floating window allows you to edit how your avatar body looks. It is activated by the top menu Edit > Appearance... item, or by pointing at your avatar in the 3D view area, right-click for context menu, and choosing Appearance. When you open the window, your avatar will go into "Appearance Mode", and will stand with arms and legs spread for visibility, and the camera will zoom to view the relevant part you are editing. You can move the camera view while in appearance mode to see how you look from other angles.
There are four body parts and nine clothes tabs, each with individual controls. These all affect your core avatar body. In addition to these, you can "Attach" up to thirty 3D objects to different parts of your avatar. For example, a backpack, hat, and walking stick if you want to look like a mountain hiker. Between the Appearance settings and attachments, you have great flexibility in what you look like.
Adjustments in appearance mode will show on the screen immediately, but will not be saved until you use the Save or Save As... buttons. It's a good idea to first make a copy of your current appearance in your inventory before editing it. That way if it gets messed up, you can go back to the original and try again.
Your avatar's shape is defined by a 3D wireframe model, as illustrated here. It consists of numerous triangles, on which the skin and clothes are painted by your graphics card. The shape tab adjusts the wireframe model using sliders or number values to change detailed parts. The entire collection of settings can be saved as an inventory item. When you "wear" the saved shape, it will adjust all the settings at once. You can create your own shape from scratch, or get pre-made shapes from other users or buying them.
There are 9 buttons to control different parts of the shape. Each button displays a set of images, sliders, and number values from 0 to 100. The images show what the extreme (0 and 100) values would do to your avatar's appearance, and the sliders and numbers show your current settings.