Many flowers are edible and many are not. Some are poisonous and dangerous so you have to be careful! A good rule of thumb is that anything that is served on a plate at a restaurant has to be edible, so if for example, your dinner plate or cocktail comes decorated with orchids, you can eat them! One special thing about flowers is that they contain phytochemicals, which are the substances in a plant that are responsible for their colors (like in the flower), immune defenses, among many other qualities/functions. The phytochemical chlorophyll, for example, is essential for the process of photosynthesis (necessary for a plant to turn sunlight into "food"). Though flowers are not particularly nutritious in terms of an abundance of vitamin or mineral content, you can check here to see the nutrition facts of a flower blossom in general (just put "flower blossom" in the main search field) when you get to the "Track Food" tab: http://www.gbhealthwatch.com/dietnutritionevaluator.php .
Sure, as long as they are not sprayed, as store bought flowers are. I ate a petal from our young peony and it was sweetly delicious. A bit later and it was bitter. Pansy, petunias, carnations, are a few. But you have to have positive i.d.s on everything as some flowers are deadly.