Today it is AJax

What is Comet(reverse Ajax)?

it is the illusion that data being pushed from the server.
for example:

a web aggregator that fetch feeds from different sources and view it to the user, the data may viewed as it is being pushed from the server using Comet

How it works ?

a separate process is responsible for fetching data from the server perodically, then it pushes these data to the aggregator