Difference of Apache and IAS_HOME

What is Apache?
Apache is webserver that Handel's http request s from client.

What is ias?
By using Apache technology oracle has customized to oracle application by adding mod_jesrv,mod_plsql and mod_oc4j.it is called oracle application server ias

What web server will do?
A webserver is responsible for take http request from client.
If URL is http://oracleoltp.com
 It will be break into 3 steps
1. Http request sends to webserver(http protocol)
2. Oracleoltp.com constants Dns to get ipadress
Of server to access the file like appslocallogin.jsp or index.xml
3. Once we get the ipaddres then it will form connection to
Server and get the file like  index.html,appslocallogin.jsp..etc

How user is able to see webpage?
Web browser-->Dns-->Ipaddress-->form a server connection-->using http protocol request for file(login.html,appslocallogin.html..etc)-->it will send html text to browser-->browser reads html text-->finally user sees the screen.

Ohs components?
Apache
Mod_jserv
Mod_plsql
Oproc
What is Apache and oacore?
Apache is webserver
Oacore is oc4j instance which works on oracle application framework


When Apache Web server up it uses the below configuration Files. 

adstrtall.sh -> adapcctl.sh ->apachectl-> httpd.conf (which in turns apache web server i.e http server will up) 

When httpd executable first starts, this file is processed. Thus httpd.conf is the main configuration file. 

httpd.conf -> jserv.conf -> jserv properties -> zone properties 
httpd.conf -> oracle_apache.conf -> ojsp.conf -> plsql.conf -> apps.conf 
httpd.conf -> oprocmgr.conf

Link for 11i application:http://mtalmasri.blogspot.com/2011/07/oracle-apps-apache-overview-11i-and-r12.html?m=1