Sorry for this newbie question

but what is the different between LOT's and BAT's?

LOTS - they are a combination of existing buildings in SC4. You can dwonload a LOT editor and combine LOTS to create your own LOTS. this is the easiest way to make your own building. (basically creating your own building by mixing existing buildings together)
file sizes are generally under 100kb and contain 1 DAT file only.

BAT- [b]uilder's [a]rethetical [t]ool. All custom buildings are BAT. e.g buildings that are not in the orginal game. like WTC, Custom ploppables.
file sizes are generally over 500kb and contain several files.