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Do you mean the underlying blockchain? Or their purpose in society?
Block chain is just a detailed record of transaction through a decentralised network.
Basically bitcoin is like a digital gold, there is a finite number of bitcoin created which can be unlocked by a process known as mining. That is bitcoin is designed to be a store of value or a commodity if you like. For this reason though, the fees on bitcoin are relatively high (for a crypto currency) and the transactions are very slow.
Litecoin is designed to be transferred with much lower fees and higher speeds. That is litecoin can provide bitcoin with the ability to be spent.
Does it make a little more sense or are you still confused?