A Bezier is a curve (or sequence of curves) described in a particular mathematical way, allowing complex shapes to be defined with fewer entities than if you were to try to approximate the shape with lines or arcs.
A series of Bezier curves are defined by control points - each curve entity having four control points. (Mathematically they are "cubic" Beziers.) Normally these control points are arranged so that each curve is tangential to its neighbours.
A Bezier will start and end at the first and fourth control points, while the second and third define its direction at these start and end points.
You can draw Bezier entities using the Bezier tools.
You can modify Bezier entities once they are drawn using the Bezier Node Editor tool. Additionally the coordinates of the control points may be modified via the Entity Inspector, but it is difficult to control the shape of the curve when doing this.
Bezier entities can be converted to lines and arcs using the Explode tool. Conversely you can use the Smoothing tool.
Many plotters, and some data formats, do not support Bezier curves - in these cases they are automatically converted to lines and arcs.
The visibility of Bezier entities can be modified via Visibility options (as can the visibility of most other entity types).