Anton van Leeuwenhoek was the person who discovered cilia and flagella in 1676. The discovery was sent to the Royal Society of London in a letter that described his findings as a second sort of animalcules whose figures were oval in shape with tiny legs.
At the time, Cilia and flagella were described as being oval in shape and having tiny legs that moved very nimbly. Cilia and flagella are actually the same, according to NCBI, but they were given different names far before their individual structures were determined and reviewed.
Usually, cells have one of two flagella, but ciliated cells are described as having many small cilia. For example, a cell can have one flagella, two flagella or thousands of cilia.
Both cilia and flagella are used to travel and to bring in food particles. They also move materials across the surface of tissues. For instance, people have cilia in their noses; these cilia cover the respiratory passages and help expel and dislodge particulates that collect inside the mucus secretions found in these passages.
Cilia and flagella move cells through beating, which can move cells as fast as 1 mm/s. Some cilia and flagella are as large as 2 mm in some kinds of insect sperm.