Flowers are sets of highly modified leaves that function to attract a pollinator or, if no animal pollinator is used, to optimize spore dispersal in some way. Over the course of evolutionary history and coevolution, this has lead to an incredible diversity of shape, size, color, smell, and just about any other characteristic you can think of. Because most plants are angiosperms and because flowers are often so diverse, learning the terminology to describe flowers is a major step in learning to identify plants.
Flowers that have multiple lines of symmetry (like a starfish) are radially symmetrical, also called actinomorphic. Flowers with only a single line of symmetry (like you) are bilaterally symmetrical, also called zygomorphic.
Radial Symmetry (Actinomorphic)
Bilateral Symmetry (Zygomorphic)
Most flowers are composed of four whorls. If all whorls are present, a flower is said to be both complete and perfect. If any whorl is missing, the flower is incomplete. If one of those missing whorls is either the androecium (pollen-producing) or gynoecium (seed-producing), the flower is also imperfect.
We can use the location of the ovary to further distinguish between flowers. If the other whorls of the flower meet below the ovary (the ovary or ovaries look a bit like an egg or eggs in a nest), the ovary is superior (on top of the rest of the flower). This means that the rest of the flower parts are below the gynoecium, so we can also call this flower hypogynous (below the gynoecium). The two terms both describe the same situation, but superior refers only to the ovary while hypogynous refers to the flower, in general.
Superior Ovary (Hypogynous Flowers)
Inferior Ovary (Epigynous Flowers)
In the opposite situation, the other floral whorls join at a point above the ovary. In this case, the ovary is inferior and the flower is epigynous (on top of the gynoecium).
Semi-inferior Ovary (Perigynous Flowers)
As always, there are less clear situations. In some flowers, as in the rose family, the floral whorls join together and fuse at a point above the ovary, then travel down, around, and below the ovary as a fused unit. This fused unit is called a hypanthium. The ovary is termed semi-inferior, as it is located below the unfused parts of the floral whorls. Because the floral whorls travel around the ovary as the hypanthium, the flower is perigynous (peri- meaning around).