Kinds are sets of properties the program uses to recognize elements in a web page. To create a Kind you must first activate selection mode by clicking on the Selection Mode 
button, select a few sample elements in the page by holding the CTRL key and clicking on them, and then click on the Create kind from selection 
button in the Kinds 
tab. You will see that a list of properties and values will appear. These properties are the properties that are common to all the elements you selected when creating the kind. You can add elements by selecting more elements and clicking on the Add Selection to this Kind 
button. This will make the list of properties shorter (and consequently include more elements) by keeping only the properties that the created kind and the newly added elements have in common.
Kinds can also be created manually from the Define kind manually
button. For instance, a kind defined by only setting the TagName property to A will select every link in any page.
To see how you can create customized property gatherers see Property Gatherers.