what is the difference between less than truckload and semi trailers?

Question posted by Rosa Manson, on behalf of Humanitarian Emergency Planning Disaster Relief Consultancy Ltd in Cullompton, Devon

I am hoping to send out a trailer load of 1570, pallet quantity 60 of 70 litre boxes of 370gms. They measure L=600mm X W=400mm X H=400mm (external) L=512mm X W=343mm X H=375mm (internal) box type: attached lids.