Let's say you are using a bar chart. The bar chart shows 3 bars. Each bar corresponds to a DataPoint. A data point is an information unit that carries information about an underlying business item. If you business class is FruitInfo with two properties (Name and Value) and in your items source you have three items:
Three data points will be constructed out of this.In order for these data points to be constructed properly you need to define item mappings. In the item mapping you can specify from which property should the YValue be extracted - in this current scenario this is the Value property. You can also define how the item is distributed along the X-axis. This is where the XValue and XCategory kick in.
Lets say you have a third property Date for this FruitInfo class.
2012/11/15 Bananas 4
2012/11/26 Carrots 5
2012/11/27 Tomatos 10
If you map the Date property to XCategory - the dates are treated as simple categories. They are being positioned right next to each other. If you map the Date property to XValue - the X-axis will distribute them according to the values that the Dates present. In this case the Bananas and Carrots bars will be far away, while the Carrots and Tomatos bars will be right next to each other.
Basically this is the difference, I have also attached a snapshot of the different output these two produce.
the Telerik team