If one web server isn't enough to handle all your traffic, you can share the load. But you need to be careful that you "maintain state"
for your visitors if you're running applications that involve a series of forms / inputs that follow on from each other.
Specialist hardware load sharing devices are available ... though often they turn out to be dedicated computers running Linux and a specialised piece of software. Such devices sit at the public entrance to your website - your front door - and distribute all incoming traffic around as appropriate. If you don't want to go to the expense of running a hardware load sharing device, you'll often find that a web server such as Apache httpd, running mod_proxy
and perhaps mod_proxy_balancer
will do just as good a job for you. After all, a restaurant with 10 waiters / waitresses only needs one maitre d'hotel to seat people and to check them out at the end, so you'll only need one front door server even if you have a lot of web servers doing the real work behind it.
There are a number of ways of sharing the load of a single website across multiple servers.
In a distributed
setup, certain folders / subdirectories are forwarded to one second tier server, other folders to another second tier server, and so on.
In a balanced
setup, NEW connections are forwarded based on a balancing algorithm to a series of near-identical second tier servers. The balancing may be as simple as "round robin" where each second tier server takes its turn to get new clients, or the top (load balancing) server may look at queue length, or even how quickly each server in the second tier is responding as it makes its choice. RETURNING (old) connections are passed back to the same server that previous requests were passed to, so that a user's series of forms will all be handled by the same machine.
In a clustered
setup, all connections are forwarded based on a balancing algorithm. However, in this case no account is taken of whether or not the connection is a new one, and requests can end up at any of the second tier servers. However, in a cluster, each second tier server broadcasts (multicasts) after each request is handled to copy the user's current data to all servers. That way, whichever server is next selected, the data will be on hand for it to work correctly. (written 2012-10-13, updated 2012-10-20)
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articlesA608 - Web Application Deployment - Apache httpd mod_proxy 
How can I run multiple web servers behind a single IP address? - (2012-04-02) 
Simplest ever proxy configuration? - (2011-06-28) 
Distributing the server load - yet ensuring that each user return to the same system (Apache httpd and Tomcat) - (2011-05-18) 
Load balancing with sticky sessions (httpd / Tomcat) - (2009-10-29) 
Apache, Tomcat, mod_proxy - (2009-08-06) 
Three recent questions on Tomcat Convertors - (2009-07-07) 
Virtual hosting and mod_proxy forwarding of different domains (httpd) - (2009-03-01) 
Forwarding session and cookie requests from httpd to Tomcat - (2008-12-14) 
mod_proxy_ajp and mod_proxy_balancer examples - (2008-12-13) 
mod_proxy and mod_proxy_ajp - httpd - (2008-08-22) 
Strange behaviour of web directory requests without a trailing slash - (2008-03-06) 
Choosing between mod_proxy and mod_rewrite - (2007-10-02) 
Sharing the load with Apache httpd and perhaps Tomcat - (2007-03-29) 
Apache httpd and Apache Tomcat together tips - (2006-12-24) 
Clustering, load balancing, mod_rewrite and mod_proxy - (2006-11-21) 
Apache httpd to Tomcat - jk v proxy - (2006-03-03)A690 - Web Application Deployment - Clustering and load balancing 
Java web application for teaching - now with sessions and clustering / load balancing demonstrations - (2015-02-20) 
Clustering on Tomcat - (2009-10-30) 
Sharing the load between servers - httpd and Tomcat - (2009-02-28) 
Automated server heartbeat and health check - (2009-01-16) 
Load Balancing - Hardware or Software? - (2009-01-15) 
More HowTo diagrams - MySQL, Tomcat and Java - (2008-08-24)
Some other Articles
An email marathonFlowchart to program - learning to program with Well HouseA wet SaturdayPublic Transport across Wiltshire - a new mapDistributed, Balanced and Clustered Load Sharing - the differenceThe components of an Apache httpd / Tomcat / MySQL stack and what each doesHere comes Santa - on the train from Melksham to Swindon on 2nd December 2012Melksham Chamber of Commerce - whence in 2013 and beyond?What a difference a year made - Melksham CampusInheritance, Composition and Associated objects - when to use which - Python example