All changes to a component are recorded in version history. The version number will depend on the workflow state that the component is in. When a component is first created its version number will be 0.1.
Saving a component
Each time a change is made to a component and saved, the minor version number is incremented and the changes are recorded in a version history.
For example, adding a field to a newly created component and pressing Save would increment the version number from 0.1 to 0.2.
Publishing a component
When a component is ready for publishing, pressing Publish in the Component builder will change the major version number.
For example, having added all my fields to a newly created component and saved it, pressing Publish increments the version number from 0.2 to 1.0 and changes the workflow state of the component to Published.