Forge allows you to load mods onto your Minecraft server. If you'd like to load Craftbukkit plugins as well, you'll need to use Cauldron. See here:

http://hosthavoc.com/billing/knowledgebase.php?action=displayarticle&id=41

The steps are as follows:

Step 1: Determine the version of Minecraft that you plan on running (this will depend on the mods that you're running, etc.). Most mods & mod packs will display the necessary version.

Step 2: Visit this page and download the !!!Installer!!! for the respective version of Minecraft you want to run on the server.

Step 3: Run the installer JAR file on your system and install Server files to a folder somewhere on your computer. We generally suggest doing it somewhere on your Desktop.

Step 4: Connect to your server's FTP server; http://hosthavoc.com/billing/knowledgebase.php?action=displayarticle&id=35

Step 5: Upload all of the contents from the folder you installed Forge to your server. The files you're uploading should look like the following:

Step 6: Rename the forge jar file to be either custom.jar if you are running MC version 1.12.X-1.16.X or custom-1112below.jar if you are running MC version 1.11.2 or below

Step 7: In the Minecraft control panel, Select either the 'Custom Jar File (MC 1.12 to 1.16x)' or 'Custom Jar File (MC 1.11.2 and Below)' options in the Jar File setting depending on the version you are running

Step 8: Click the Save button to save the changes.