Bukkit server Mods

You can install mods, most bukkit servers run plugins. Plugins are server-side mods, not effecting the client. Mods are client-side, not effecting the server but instead the client. Plugins can be used for minigames or new world generators (using vanilla blocks). Mods can create new items, and more. Do what TwistedReality said to install a plugin, to install a mod, I strongly recommend you google it instead of asking for help, as installing mods on servers can be very difficult.

If you really mean mods you can use MCPC+ instead of bukkit (it is like bukkit it but all has ability for mods)

I assume you mean plugins?? If so, You install them by dragging them to your plugin folder, then reloading or restarting your server console.