A butter churn is a mechanical device used to agitate milk cream until it becomes butter. Churning the cream brings its fat globules together and causes them to clump into lumps of butter, leaving a liquid called butter milk.
The physical phenomenon which occurs is called phase inversion - the inner phase and outer phase of an emulsion exchange places: oil in water emulsion (cream) becomes water in oil emulsion (butter).
Many configurations of churns were used throughout history. The end-over-end churn stands about high and swings on a central axle. The "barrel churn" was similar but rolled on its long axis. The plunger-type is harder work than the end-over-end or barrel churn. A plunger or dasher has to be moved vigorously up and down by hand.
The upright plunge or dash churn is the earliest kind of churn. There is evidence that this type was in use by C. 600 AD. The barrel churn was one of the agricultural innovations of 18th century Europe.