How to create curved objects?

How do you create curved parts like this image above? PLease briefly tell us your methods! Thanks!

Use curve objects.

Your image is made up of two objects. A line and a circle. In Blender terms, the line is the path that the circle will be deployed along. Create the line (SHIFT-A Add BezierCurve). Then create a Circle (SHIFT-A Add BezierCircle, not mesh circle-won’t work). Select the line and activate the data context. From there you can specify a control shape for the line. This is called the “Bevel” object. Click the Bevel object drop down and you should see the Circle in the list.

Alternately if you only want a circular tube you can just increase the Bevel Depth of any line/curve to make it render in a tube like fashion. The resolution at that point is controlled bu the Bevel Depth (i.e. number of points to define a circle)