At the ftp site you linked to you have five options:
CentOS-5.3-i386-LiveCD.iso - This is a Live CD, which means if you boot your PC with this, you can use CentOS without actually installing it to the hard drive. It will load a graphical workstation setup. However, any programs you install or settings you change will not be saved, unless there is a utility to save them (I'm not too familiar with the CentOS LiveCD.
CentOS-5.3-i386-bin-Xof6.iso - X being 1 - 6, these are CD ISO images to download and install with. An absolute minimal install will require just the first disk, but a standard install will require at least 1 - 4, if not all (I can't remember which are needed for a default install). Note than when I say absolute minimal, I mean that it doesn't even install a GUI. The OpenSSH server is installed though. This is a great way for an advanced user to setup a server with a minimal footprint, with as few services installed as possible.
CentOS-5.3-i386-bin-1to6.torrent - This is a bittorrent file for downloading the entire 6 CD ISO set at once. This is just an alternate way of downloading the CDs and is an example of how bitorrent can be a good technology for redistribution.
CentOS-5.3-i386-bin-DVD.torrent - This is a bittorrent file for the DVD installer ISO. Unlike the CDs, you only need the one DVD to install everything.
CentOS-5.3-i386-netinstall.iso - This ISO image contains a minimal setup that will allow you to download the packages and install them. It doesn't contain any packages itself, it must be used while connect to a network or the internet. If you have a network setup that has all of the updated packages installed, this is a great way to get a fully up-to-date system installed. Not really for single install use though.
Some mirrors will have a sixth option:
CentOS-5.3-i386-bin-DVD.iso - the DVD ISO image itself instead of the torrent. Few mirrors carry it though due to the burden of hosting a DVD image.
The md5sum and sha1sum files are used to verify the ISO downloads to make sure they are not corrupt.