Realtime access app

Want to understand the basis how does online app like mapquest, google map can support so many users simultaneously?
Won't the servers running these app get bog down by the sheer number of users in real time?
Or the users actually downloaded an applet onto their computer when accessing mapquest, googlemap etc when they run a search for driving direction?

Thanks in advance.
S101

The users do not download an applet. Google can support a lot of users because they have a lot of servers. Exactly how many isn't public knowledge, but it's thought to be over one million.