Just like bitcoin, litecoin is a crytocurrency that is generated by mining. Litecoin was created in October 2011 by former Google engineer Charles Lee. The motivation behind its creation was to improve upon bitcoin. The key difference for end-users being the 2.5 minute time to generate a block, as opposed to bitcoin's 10 minutes. Charles Lee now works for Coinbase, one of the most popular online bitcoin wallets.
For miners and enthusiasts though, litecoin holds a much more important difference to bitcoin, and that is its different proof of work algorithm. Bitcoin uses the SHA-256 hashing algorithm, which involves calculations that can be greatly accelerated in parallel processing. It is this characteristic that has given rise to the intense race in ASIC technology, and has caused an exponential increase in bitcoin's difficulty level.
Litecoin, however, uses the scrypt algorithm – originally named as s-crypt, but pronounced as 'script'. This algorithm incorporates the SHA-256 algorithm, but its calculations are much more serialised than those of SHA-256 in bitcoin. Scrypt favours large amounts of high-speed RAM, rather than raw processing power alone. As a result, scrypt is known as a 'memory hard problem'.
The consequences of using scrypt mean that there has not been as much of an 'arms race' in litecoin (and other scrypt currencies), because there is (so far) no ASIC technology available for this algorithm. However, this is soon to change, thanks to companies like Alpha Technologies, which is now taking preorders.
To highlight the difference in hashing power, at the time of writing, the total hashing rate of the bitcoin network is over 20,000 Terra Hashes per second, while litecoin is just 95,642 Mega Hashes per second.
For the time being, 'state of the art' litecoin mining rigs come in the form of custom PCs fitted with multiple graphics cards (ie: GPUs). These devices can handle the calculations needed for scrypt and have access to blisteringly fast memory built into their own circuit boards.
There was a time when people could use GPU mining for bitcoin, but ASICs have made this method not worth the effort.