when i try to some rpm file then it isn't work; it shows like:

[root@localhost Software]# rpm -Uvh kdemultimedia-extras-3.5.2-5.fc5.i386.rpm
warning: kdemultimedia-extras-3.5.2-5.fc5.i386.rpm: Header V3 DSA signature: NOKEY, key ID 1ac70ce6
error: Failed dependencies:
kdelibs >= 6:3.5.2 is needed by kdemultimedia-extras-3.5.2-5.fc5.i386
kdemultimedia >= 6:3.5.2 is needed by kdemultimedia-extras-3.5.2-5.fc5.i386
libakode.so.2 is needed by kdemultimedia-extras-3.5.2-5.fc5.i386
libgstreamer-0.8.so.1 is needed by kdemultimedia-extras-3.5.2-5.fc5.i386
libtag.so.1 is needed by kdemultimedia-extras-3.5.2-5.fc5.i386
libtunepimp.so.3 is needed by kdemultimedia-extras-3.5.2-5.fc5.i386

OR

[root@localhost Software]# rpm -Uvh xmms-1.2.10-23.fc5.i386.rpm
warning: xmms-1.2.10-23.fc5.i386.rpm: Header V3 DSA signature: NOKEY, key ID 1ac70ce6
error: Failed dependencies:
libxmms.so.1 is needed by xmms-1.2.10-23.fc5.i386

CAN ANYBODY TELL ME WHY?

Because it says right there. You need to install dependencies (other rpms) because the one you are trying to install depends on some more rpms that are not installed. It is better you install using YUM.

Tux,

thanks tuxrules. yeah i've seen that yum has good facalities. but i can't use net from linux. and i haven't found any linux sopported modem yet in bangladesh. can you tell me one thing that Prolink V.92 tornado external modem is support by linux?

Not a problem...
have a look at www.linuxquestions.org/hcl/