Yum maintains a cache of the RPMs it downloads here:
/var/cache/yum/<you architecture>/<OS version>/
For example my box is Fedora 14, x86_64 architecture so I have the following:
$ ls /var/cache/yum/x86_64/14/
adobe-linux-i386 lamolabs rpmfusion-nonfree
adobe-linux-x86_64 lamolabs-noarch rpmfusion-nonfree-debuginfo
Dropbox nautilus-flickr-uploader rpmfusion-nonfree-rawhide-debuginfo
fedora qm rpmfusion-nonfree-source
...
...
But the RPMs that get installed are basically like a zip or tar file. So the contents of these files get dumped into the system and RPM maintains a database of what packages it has installed. The RPM "database" is located in this directory:
$ ls /var/lib/rpm
Basenames __db.002 Dirnames Installtid Packages Pubkeys Sha1header
Conflictname __db.003 Filedigests Name Providename Requirename Sigmd5
__db.001 __db.004 Group Obsoletename Provideversion Requireversion Triggername
You can see where files get installed per each RPM with these commands.
what files are in an installed RPM
$ rpm -ql rpm
/bin/rpm
/etc/rpm
/usr/bin/rpm2cpio
/usr/bin/rpmdb
/usr/bin/rpmquery
/usr/bin/rpmsign
/usr/bin/rpmverify
/usr/lib/rpm
/usr/lib/rpm/macros
/usr/lib/rpm/platform
/usr/lib/rpm/platform/amd64-linux
...
...
package info about a given RPM
$ rpm -qi rpm
Name : rpm Relocations: (not relocatable)
Version : 4.8.1 Vendor: Fedora Project
Release : 7.fc14 Build Date: Tue 04 Oct 2011 03:49:08 AM EDT
Install Date: Tue 25 Oct 2011 09:36:34 AM EDT Build Host: x86-05.phx2.fedoraproject.org
Group : System Environment/Base Source RPM: rpm-4.8.1-7.fc14.src.rpm
Size : 2035701 License: GPLv2+
Signature : RSA/SHA256, Tue 04 Oct 2011 12:14:48 PM EDT, Key ID 421caddb97a1071f
Packager : Fedora Project
URL : http://www.rpm.org/
Summary : The RPM package management system
Description :
The RPM Package Manager (RPM) is a powerful command line driven
package management system capable of installing, uninstalling,
verifying, querying, and updating software packages. Each software
package consists of an archive of files along with information about
the package like its version, a description, etc.