I have been hearing about MODs for a long time but i stll dont know what it is. Can anyone tell me what it is?

From the Forum FAQ:

What is a Mod?

A mod, in the general sense, is literally modifying a game to suit your needs. Some games are built to be modified, others have to be hacked. Some even come with their own tools to create new levels and campaigns. Whatever the case, a game is considered modded if any of its default properties are changed, whether by switching weapon models, adding new levels, extending the story with new campaign or adding new gameplay elements. And those are just a few examples.

For instance, we have StarCraft, which ships with a map editor that allows you to build your own interactive maps for multiplayer. The modding comes in when a person creates a campaign that extends or builds on the story of the game (which some have done). The same is true for games such as Deus Ex and Half-Life.

However Deus Ex and Half-Life (along with Homeworld) provide a higher form of modding, that is, changing the game"s actual attributes. For instance in deus Ex we can find people creating an entirely different game with new weapons, items, etc. The same applies for Half-Life (as its popular Counter Strike mod shows) and Homeworld(they"re even making a Star Wars vs Star Trek Homeworld by replacing the normal ship models with those from the sci-fi movies!).