Welcome to the hive.
Your questions are more suited for World Editor Help Zone, here:
http://www.hiveworkshop.com/forums/world-editor-help-zone-98/
(1) Models are 3D objects that can be used in Warcraft III. Our site has plenty of custom ones. You can import them into your map and use them. Just check out the model section, and use the "View in 3D" button if you want a good preview!
(2) Skins are textures applied to models. For example, you can make a sorceress skin completely green, and then importing it will turn your sorceress' green. Each model has its own texture, and it controls the colors/exterior of the model. The model is just the geometry. The texture actually adds all the colors/details, see:
Most models will have textures. Even the custom ones that don't include textures will use textures (they'll use ones in-game, so you don't have to import new ones).
(3) Packs are just packs of models and skins (or even maps/spells). When a user submits a model, it will be reviewed individually. They might make a pack of trees or something, but they have to upload them separately. Downloading them all may take a long time, so packs will consolidate them so you can download them in one zip file.
Anyway, the first thing to do is to check out the tutorials section:
http://www.hiveworkshop.com/forums/tutorials/
It will be your number 1 reference and guide to all sorts of things. There is also our affiliate's site here:
http://world-editor-tutorials.thehelper.net/
Which is popular, especially for a lot of editor basics. If you get stuck on anything, feel free to make a thread in World Editor Help Zone, and someone will answer.
Good luck!