I have never heard anyone refer to Ageratum as anything but Ageratum but catalogs sometimes say floss flower so I will include that name, too. You see Ageratum sold everywhere in the spring in packs as a bedding annual. The flowers are usually powder blue with a hint of purple, but there are white, purple and pink varieties, too. Each flower is made up of many short, thread-like petals. The flowers are only a 1/4 or a 1/2 inch across but are held in clusters that are 2-3 inches across. The overall effect is soft and fuzzy. You can see them dead center in the picture below.
The bedding varieties only grow 6 or 8 inches tall but there are also a few tall strains of Ageratum that grow a 18-24 inches tall. The most common cutting varieties are 'Blue Horizon' and the 'Dondo Series' which comes in blue, violet and white. There's also a reddish purple one called 'Red Sea'. All of them are easy to start from seed but are tall enough that they usually need support.
I grew the white ones once and found they quickly discolored so I have not tried them again. I also tried 'Red Sea' and didn't care for it. The buds are more red and the flowers more purple but you see both in the same cluster. It looks like it can't decide which color it wants to be. The color also somehow loses some of the softness that I like in the blue, so for now 'Blue Horizon' is the only one I grow.