user-controls = controls that are designed and coded by user.
user-controls are used for purpose of re-use design & functionality.
there are different base controls available in .net
eg. label, grid, panel, textbox, radio button etc.
user-controls are controls which have generally more-than one base controls inside.
and perform some kind of task with/without different parameters/properties
eg.
suppose, for a manufacturing company you are developing ERP,
you need calculator to calculate 'volume of 3d-rectangular job'.
suppose you will need this cal-c in all production forms & invoice forms
then for achieve this re-usability
create usercontrol,
that have 3 textboxes for input l-b-h and 4th label for display result.
now, code it
and build your application you will get this user-control in toolbox
now, you can use it in different forms by just drag & drop it to your form.
you can create different properties & methods of usercontrol.
Happy coding!
:)