iCalendar is an Internet specification for calendar information sharing that
can be implemented in a number of ways to meet the requirements of different
applications. In a typical web application, the iCalendar information is
generated on-demand from a database and presented as the appropriate MIME
type to the user, who sees it in whatever UI his/her iCalendar-capable
application displays it. An Outlook user would see a new appointment and can
click Save to save it. A web search for "iCalendar
ASP.NET" should turn up
lots of useful information for you.
Adding a calendar item directly to a user's Calendar through client-side
Outlook automation generally is not possible, because of ActiveX and other
Outlook security. An alternative would be to create a downloadable component
that the user could install that your page could interact with.