Custard describes a whole range of different dishes that are all made from a base of either cream or milk and egg yolks, with other ingredients being added as desired. Custards can vary from a thin sauce that is poured over food to a thick cream that is used to fill pastries, and they can be used for both savoury and sweet dishes, either as a base for other ingredients to be added to or as a filling itself.
In this case, it's a sweet dish, to which vanilla has been added. Vanilla is a very expensive spice (only saffron is more expensive) that is derived from a species of orchid. Given the cost of true vanilla, it isn't uncommon for substitutes to be used instead to reduce costs. Although a bowl of custard could be eaten as-is, it is more common to use it as a component in a larger dish.
Although traditional custards all use egg yolks and, indeed, the term shouldn't be used for those that don't, there are a number of modern preparations called custards that are egg-free. These are often available in a powdered form, to which milk is added, and which are then heated.
