How does QTP Descriptive Programming work?

First of all, I've created new Description object: 

Set Desc = Description.Create()

Description object contains collection of properties, which identify any UI object such as a browser, a page, a dialog, a list, a button etc.

To specify that we want identify all Edits on browser's page I use "micclass"property:

Desc("micclass").Value = "WebEdit"

Note: the "mic" prefix in "micclass" stands for "Mercury Interactive Constant".

How do you know the class name ("micclass") of object?

Use Spy for that:
Open QTP object Spy and check recorded properties of object.
For example, these are properties of Edit:
As you can see, there is "Class Name" property and its value - "WebEdit". So,"WebEdit" is a value of Class Name of all Edits located on Web page.
Note: "Class Name" is a synonym of "micclass". 

I gathered Class Names of Web objects in this table:

#	Type of Web object	Class Name(micclass)
1	Web Browser	Browser
2	Page	Page
3	Edit box	WebEdit
4	Image	Image
5	Link	Link
6	Web Element	WebElement
7	Button	WebButton
8	Checkbox	WebCheckBox
9	Combobox (DropDownList)	WebList
10	Table	WebTable

Since we created Description object for all edit boxes, we can use this description to get all specified objects ( = edit boxes).
The next step returns the collection of all child objects (i.e. edit boxes) contained within the page:

Set Links = Browser("Google Sets").Page("Google Sets").ChildObjects(Desc)

To get the number of found objects in a returned collection, we use Countproperty:

MsgBox "Number of Edits: " & Links.Count

And the result is 5 found Edits on Google Sets  page: