The Ambrosia genus belongs to the aster family. Don't get your hopes up, though, upon hearing that name: common ragweed blooms, as you can see from the photo above, look nothing like those of New England aster, for example. The flowers are exceedingly inconspicuous. In fact, the casual observer may not even recognize them as flowers (they're the yellowish little bumps that you see in the picture).
To be more specific, the photo above shows you what the flower spike of the male flowers of common ragweed look like. Using a magnifying glass, you would be able to detect the presence of five stamens on each of these male flowers.
This weed is monoecious (see What's the Difference Between Dioecious and Monoecious?). The female flowers are even easier to overlook and can be found (if, indeed, you're determined to find them) in the upper leaf axils (i.e., the angles between the upper side of a leaf or stem and the supporting stem or branch).
The bloom is succeeded by a fruit that is technically designated an "achene." It is brownish and, like the flowers, inconspicuous.
On Page 4 we'll have another look at a photo of the whole plant -- but, this time, later in the summer, when it is in bloom....