An attachment is a prim or object that is attached to (and detached from) an avatar at various "attachment points". Any object/prim can be attached via the "attach" command in its pie menu (if in-world) or its right-click context menu (if in inventory). To detach an object, right-click it and select "Detach" from the pie menu.
There are 30 total visible attachment points and 8 HUD attachment points which are only visible on the resident's screen. Some residents have made prim shoes or hair (and other items). While these are possible to make using the appearence window, it has been found that making them with prims not only gives them more dimension but also gives the creator the ability to put more detail into them. Attachments can contain scripts that enable them to change their appearances or properties. Some properties include color, bling and the ability to "say" various statements.