Alright i am bit new to Linux(just a fortnight experience).What i guess is that there are two ways of installing softwares in Slackware(i am using Slackware 11.0)
the first method is using binaries i.e one can download Slackware packages form Slackware Site and the install them using the command installpkg pkgname.tgz
The other method is compile from source.
Now i have already tried the first method(i got a full installation i.e every package but during installation it showed that certain packages were unable to install as they were corrupt).So what i did was that i downloaded those packages(they all had the extension .tgz)and then i used the coomand installpkg pkgname.tgz.

My question is that when i see a package with the extension .tgz can i install that package directly using the above command( i guess only if that package is downloaded from officail Slackware site) and are all .tgz files binaries ???

Now i want to try the next way to install i.e from source so what i plan to do is that download the latest gcc(4.1.2 as of now)and install it from source(As far as i know the latest gcc version is not available in current Slackware 11.0 )so i plan to install it on my on from source .i have downloaded the file with extension .tar.gz now kindly tell me that should i read the instructions and install the package(i already have a deafult gcc version that comes with Slackware 11.0 installed)

kindly guide me

The Slackware site is definitely not the only place you can download binary packages. There are other sites, such as LinuxPackages that maintain a repository of user-created packages.

As for installing packages from source, you really shouldn't start with gcc. You should start with smaller and less critical applications that won't break your system if the compile/installation does not go well.

It is a matter of opinion, but many people feel (myself included) that you should not just install things from source without some method of managing the installed files. The best way to do this is to create proper Slackware packages out of the software you compile. There are a number of programs to do this, the most popular (and arguably easiest to use) being checkinstall.

It's also worth pointing out that sometimes source code is packaged as a .tgz. You don't see this very often but if you do try to use installpkg on one it will stop with an error and you should treat it like a tar.gz.

If you have never compiled an application from source before, I would advise you not to start with gcc. Compiling gcc is likely to be complicated, and there will probably be a number of options to choose during the configuration that could cause problems if they are set incorrectly. Because gcc is part of the tool chain used to compile the other packages already on your system, there are other reasons as well that it probably shouldn't be your first choice as an application to learn on.

My : You would be better served by first compiling something more straight-forward.