My english is not good so i can't understand it well when i read about it in a host website.
Please could you explain what is it !
It's mean your files are hosted in 2 servers and if one fail to respond the second works?
Let's just imagine if you need to carry 2 carton of beers. You can hold both in one hand or you can carry one on each hand (One on the left hand and one on the right hand). The second method is a simple example of load balancing It lighten the load of one hand so it does not need to carry something too heavy.
A cluster in this case would be several servers all linked together to do that one job.
I thaught it's allow to apply instant backup so if the first server is down for some reason the second works like if it is the main server.
What you're referring to isn't load balancing per se but active-passive failover. This is generally used for lower end database applications that don't require true clustering support. While it is generally easier to implement an active-passive cluster than an active-active cluster, the flipside is that active-passive has poor resource utilization since one or more machines are pretty much sitting idle the vast majority of the time. Since HTTP requests are simpler and don't require the transaction safety/syncronization that a database does, if you have 2+ machines you might as well do load balancing and take advantage of both machines to ease the burden on any single machine. Any load balancing algorithm should be able to quickly remove a node once it realizes it can no longer service requests.
I personally don't like doing failover at the external DNS level, especially if uptime is critical. Many caching/proxy servers will ignore very low TTL values altogether and rewrite those with a much longer TTL, and then there is client side DNS caching to contend with as well. So it's more of a crapshoot that just using a load balancing appliance/switch like a Cisco LocalDirector, Nortel/Alteon, F5, CoyotePoint, etc. or some Linux box(es) running Linux Virtual Server, Linux High-Availability, etc.