java - JDK/JRE explanation of comands for install

Maybe this is a simple question.

I don't find the explanation of the commands for install Java JDK and JRE

I downloaded the jdk-7u5-linux-x64.tar.gz and jre-7u5-linux-x64.tar.gz,
I extracted these files and then moved to /usr/lib/jvm/. Whell I guess that the name /jvm (I created that beacause I don't had it) is not important beacause the installation is by link.

Most webs tell me that I have write that but they not explain why.

I guess that this create a java's link in /usr/bin/

/usr/bin/java -> /etc/alternatives/java -> /usr/lib/jvm/jdk1.7.0_05/bin/java

sudo update-alternatives --install /usr/bin/java java /usr/lib/jvm/jdk1.7.0/bin/java 1

/usr/bin/java ==================> is the final link

java ========================> I don't understand why I have to write that

/usr/lib/jvm/jdk1.7.0_05/bin/java ==> original file

1 ===========================> why 1? some people write 2, 3 or I saw 2000!

the same in java, javac and javaws. And the same in JRE, but when I install JRE, th link is replaced

sudo update-alternatives --install /usr/bin/java java /usr/lib/jvm/jre1.7.0/bin/java 2

and when I choose sudo update-alternatives --config java I can only choose one, so what is the difference between jdk/bin/java and jre/bin/java (they are the same version)

Can anyone explain me that in a simple way? because I'm beginner in that, I don't know nothing of java, I only need these javas for other program that use it

Can anyone send me a good page for the correct installation of both, please.

Thanks