What is the simplest code to auto update content in the web browser without reloading?

What is the simplest code to auto update content in the web browser
without reloading?

I'm looking for the simplest code I can add to any text field or other
content on a web page that will update its value in accordance with the
response of a web service such as REST. Either JavaScript, jQuery or
WebSockets. Flash, native, etc... are cool too.