1 LTC equals 100,000,000 (one hundred million) litoshis. The litoshi is the smallest indivisible unit on the Litecoin network and mirrors Bitcoin's satoshi.
A photon (sometimes "lite") is 100 litoshis, or 0.000001 LTC — the Litecoin equivalent of Bitcoin's "bit" or μBTC. It's rarely used by wallets but appears in Litecoin community usage.
mLTC is millilite — 0.001 LTC, or 100,000 litoshis. Sometimes used by exchanges and tipping services for medium amounts.
Although Litecoin and Bitcoin use the same 10^8 split, the unit names diverged early in the project. The protocols are independent — 1 LTC and 1 BTC have no fixed exchange rate.