Most of the charts show a point mark for each data point. RadChart allows you to configure the appearance of these point marks by changing their shape and colors. To do that, you have to use ISeriesDefinition.Appearance.PointMark property which is of type PointMarkAppearanceSettings where you can change the Fill, Shape, Stroke and StrokeTickness properties of the point marks.
RadChart supports eight predefined shapes that you can use to customize the appearance of the point marks for certain series like Line, Spline, Area, etc. The shapes are members of the Telerik.Windows.Controls.Charting.MarkerShape enumeration:
To learn more about customizing the point marks read the Styling the Point Marks topic. In case the provided customizations are not enough, you can completely re-template the point mark. See here for more information.