Basting a technique for moistening the surface of roasting meat, poultry or other items, with pan drippings, stock, butter or some other liquid. In addition to contributing moisture, basting adds flavor (at least to the extent that the basting liquid is flavorful) to the surface of the meat.
Basting is usually accomplished by using a basting brush to apply the liquid to the meat. This is accomplished by dipping the brush into the liquid at the bottom of the pan and brushing it onto the meat.
A basting bulb is another way of doing this. Like a large eyedropper, a basting bulb is perhaps more efficient than a brush at extracting the liquid from the bottom of the pan, but not necessarily better at redistributing it onto the surface of the meat. So there is a tradeoff.
Ultimately, I prefer a brush, mainly because it's easier to clean than a bulb. The brush is also more versatile, since you can use it as a pastry brush too. Whereas there are no additional uses for a basting bulb that come to mind.
You can also use a basting spoon, otherwise known as a spoon.
Moreover, some chefs prefer not to baste at all, because it requires opening the door of the oven each time, which lowers the oven temperature and slows down the cooking. There is certainly merit to this argument.
Something else to consider is what characteristics want the exterior of your roasted meat or poultry to have. In the case of a roasted chicken, a crispy skin is arguably one of the main reasons for roasting it in the first place, versus cooking it in some other way. Therefore anything you do that might tend to interfere with the crisping of the skin might be counterproductive.
In some ways, basting is one of those things that people think they need to do in order to be "cooking," like stirring a soup while it simmers, or flipping and reflipping steaks or burgers on the grill. While it may appear to have a purpose, this sort of busywork is mainly a way for the cook to release nervous energy, and is not only unhelpful, but actively hinders the production of good food. The reality is that a great deal of cooking involves drinking a glass of wine in the living room while the food cooks all on its own.
Another theory around basting is that it hails from an era when pork was customarily cooked to a point that we now consider to be overcooked. The resulting dry pork no doubt led many cooks to believe that they had to do anything in their power to preserve moistness.
The reality, of course, is that spilling a bit of fat or liquid over the surface of the roast has no effect on the moistness of the interior. Truth be told, a careful cook can achieve fine results when roasting meats by selecting good cuts of meat with plenty of marbling, and striving for 145F for pork roasts and 135F for beef. See how to roast meats for more info.