Boiling Onions are normal onions that are harvested when they are still young and small. They will have pretty much the characteristics of what their fully grown selves would have; but most are pretty sharp in taste, which mellows as they are slowly cooked. Some are even sharper than others.
They are allowed to grow a bit larger than pearl onions, up to 1 1/2 and 13/4 inches wide (3.5 to 4.5 cm.) They can have white skin, yellow or red skin.
Though any onion can be harvested early and called a Boiling Onion, some varieties of onions are preferred commercially. For white-skinned Boiling Onions, for instance, Southport White Globe is a preferred variety. For yellow ones, Australian Brown, Stuttgart, or Yellow Ebenezer.
However you are cooking them, cook until soft.
If you are peeling a lot of Boiling Onions, which are small and fiddly, you can speed up the process by "cold shocking" them. Cut off the tops and the bottoms, then blanch them for up to two minutes, then drain, and dump into very cold water to stop the cooking. When they are cooled, you can just squeeze them to pop them out of their skins. Before cooking with them, make a very small X about 1/4 of an inch (1/2 cm) deep at the bottom of each onion: it will help to hold them together during cooking.