First of all, I am not a techie, but I have been involved with bitcoin, litecoin, and many other alt coins for a few years. The biggest difference would be the speed and fees charged for processing, with Litecoin being faster and cheaper. Litecoin also just recently (in the past week or two) adopted Segregated Witness (Segwit). You can google it to find more info from much more intelligent people than me.
Bitcoin has been in a battle to decide how to speed up processing and reduce fees and their are two distinct groups, one of which is proposing SegWit. So it could be argued that Litecoin is a test case for one of the future paths of Bitcoin. Again, these are just my thoughts and opinions.