I'll do my best to answer some of your questions in relation to my organisations set up
1) Suppose my first request went server1 and will it be possible to go my subsequent request to different server Its possible depending on what and how you have set up your load balancer. Some load balancers are just Ip sprayers ,however for web applications that use sessions to store state you probably want a web server such as IBM HTTP Server (IHS) or Microsoft IIS that will route requests to the same server that it originated from and if that server is down route it to another server in the cluster (you'll need to have session replication between servers for the failure scenario).
Websphere has an option to generate a web server plugin file called plugin-cfg.xml file. This file has a list of the servers in your cluster their ports and a server identifier. The server identifier is typcially part of the JSESSION cookie id sent in the request. This plugin-cfg.xml can be installed on the web servers . See Install plugin on IIS The web servers can then figure out how to direct the request based on the unique server id sent in the JSESSION identifier. If the requests are stateless (like some web service requests) then the web server picks a server based on a routing algorithm.
This excellent tech note explains the algorithm for IIS and IHS Understanding IBM HTTP Server plug-in Load Balancing in a clustered environment
2) What are the different load balancing techniqes we have
See answer 1.
Also if you want a just IP sprayer or network dispatcher then look at documentation on IBMs edge server or f5 load balancer. I have not used F5 but I have heard it mentioned in some infoq architecture presentations.
3) What is clustered environment This is probably not the correct term but for me a clustered environment typically consists of more than one application server logically grouped together. There are operational efficencies to be had with logically grouping servers such as you can deploy one application across multiple servers and configuring fail (over see answer 1).