Custom Ads. These are the most restrictive, as you select the actual items to be displayed. They work like a Search Ad, but instead of just posting all the search results as your ad, there is an extra step where you select particular items from the search, and only those will be displayed in the order you select them. I use this a lot if I'm reviewing a particular product or brand of product. I can then use a Custom Ad to offer up the item in it's different versions - colors, sizes, models, etc.

With any of these ad styles, you can choose a Grid Layout (my favorite) or a List (just a text-based format that I don't like at all), or a Strip Layout that allows the items to scroll from the side.