What is Java Applet equivalent in .NET?

Hi,

What is Java Applet equivalent in .NET?

Overview on Java Appets:
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What is an Applet in Java?
Applet is a Java program. This is referenced in a Web page
(html, jsp etc.). On being referred, the Applet (class file) is made available on the client and is executed in the client. Note that the Applet even though is hosted on the Server, necessarily runs in the client. This needs a Java Runtime (JRE) to be available in the client (browser environment) also.

Features:
- An Applet, since it runs in the client, can access the client
machine (meaning read from the file system, create files etc. in the client machine).
- An Applet can talk to the server, where it came from and not other machines.
- An Applet can launch exes in the client machine (say word exe or others)
- Since these features tend to be malicious, there is Security
attached to Applets, that comes in the form of Signing and Certificates.

If similar results were to be achieved using .NET, what is the answer?

Hope to get valuable inputs.

- Chethan