There's often no real difference between baking and roasting, except as to how the terms are used. Both refer to cooking with dry heat that is indirect -- that is, the food is not directly exposed to the heat source; rather, the air in the oven is heated by a flame or electric element to the desired temperature and allowed to circulate around the food.
Baking usually refers to breads, cookies, cakes, and so forth, but also to containers of food (e.g., you bake a casserole or a chicken).
Roasting in an oven usually refers to meats and poultry, sometimes implying a higher temperature than baking -- but the differences are pretty vague and vary depending on the speaker.
Roasting may also refer to cooking in other ways, such as on a spit over an open fire. Again, the meaning depends on the speaker.
Broiling in an oven is cooking directly under the heat source. Most ovens have a separate heating element on the top, inside the oven, for broiling (the regular heating element is usually under the oven cavity). The meat being broiled is placed under this top heating element, usually very close to it, and the heat is usually very intense. The results are similar to grilling -- which also is sometimes referred to as broiling.