A drink made with ice-cream and a fizzy soft drink (soda in North America, hence the name). Generally served in a big glass, about the height of a large milkshake from McDonald's (I call them spider glasses (because spiders are served in them), but this might be a bit of a circular definition.)
If you intend to make your own, the soft drink goes in last, and be careful when you add it: the drink fizzes up, and may overflow.
Often also called spiders, but these contain ice-cream topping and are only made with lemonade.
Apart from spiders, there are also root beer floats, which I understand, are basically root beer ice-cream sodas.