Litecoin (LTC) is the very first alt-coin, and was originally designed to be the "silver" to Bitcoin's "gold" standard.
For a while the prices of both were quite closely linked. However, since 2015, due to some development changes, it has been trading more independently from Bitcoin.
The main feature - and difference from Bitcoin - is Litecoin's use of the Scrypt algorithm as its proof-of-work process. Unlike the SHA-256d algorithm it inhibits the scalability of hardware by requiring a significant amount of memory while running calculations. This serves to slow down the economic incentive to start developing custom hardware solutions by making it less economical to do so.
Another significant difference from Bitcoin is a targeted transaction time of 2.5 minutes - as opposed to the lengthier 10-20 minutes required to confirm a Bitcoin transaction.