Modding / Plugins
.Mod(ifications) Are for singleplayer. Plugins Are for multiplayer.

.
Modelling
. It is making a model for a mob or item that is 3D and in your minecraft mod/plugin

.
Skin Making
.It is the term for a texture on a mod they can also be in texturepacks but they can be the player itself or another entity (E.G.villager/pig/enderdragon) there may be multiple skins for 1 type of mob in use if you have the random mobs mod installed. a skin will fit on a model and each side is a seperate chunk of skin

.
Texture Packs
.it is they way to change the look of minecraft but it must be up to date otherwise newer blocks wont appear to be there even if they are there

.
Progam's