The clue is in the name – X11 algorithm is basically a mining algorithm which takes eleven other algorithms and stitches its altogether into a single mining algorithm. Its constituent algorithms are:
X11 was originally created because its developers wanted to move past the difficulties of the classic algorithm used by many cryptocurrencies – think SHA256 for Bitcoin, or Scrypt for Dogecoin and Litecoin. The reason these mining algorithms became problematic is because ASIC miners started to take over mining for the coins powered by these algorithms – making it difficult for everyone else to effectively mine using standard hardware. As a result the coins became more centralised.
The fact that X11 uses so many different algorithms in its composition means that ASIC miners cannot really mine X11 – because ASIC miners are good at very specific tasks only, like mining a specific algorithm. Another benefit is that X11 can hash quickly using CPUs and GPUs, while ensuring that GPUs stay cooler – compared to the hot temperatures generated when mining SHA256 or Scrypt. All-in-all X11 was intended to decentralise cryptocurrency mining.
Dash is the most popular X11-powered coin. It was previously also known as Xcoin and DarkCoin. Even though Dash has changed name several times over the years it is now known as a cryptocurrency which offers its users instant transactions, private transactions and greater decentralisation too – with both budgeting and governance less centralised than other coins. Dash also has a highly active and engaging community.
Other coins also make use of X11 but Dash is by far the most popular of the X11-driven coins. One example is Startcoin which was intended to help move crowdfunding ahead, in part by supporting change. Another X11 coin is Digitalcoin which says it makes sending money more private, faster and more secure. Spreadcoin has an X11 variant called SpreadX11 that has an interesting tweak to X11: it prevents the ability to pool mining power. The idea is to maximise the degree to which a coin is decentralised by distributing mining activity.
X11 has a major benefit: individuals can still use standard computers with a fast CPU and GPU to mine X11 coins like Dash. There are few ASIC mining rigs available that are able to effectively mine X11, reducing the competition for mining X11 coins. There are ASIC miners that can mine a coin like Dash but the increasing mining difficulty of Dash has meant that these ASIC miners very quickly lost their use.
There is another benefit to X11 when you compare it to an algorithm like SHA256 or Scrypt: X11 is overall less hardware intensive which means less electricity is consumed when mining an X11 coin. It also implies that GPUs are about 30% cooler when mining X11, compared to using a GPU to mine another algorithm.
If you want to effectively mine X11 coins you need to have a rig with a powerful CPU and GPU. The Dash website has a page which will explain what you need to do to put together an X11 mining rig. The site will also give you hash rates for specific GPUs so you can work out whether your choice of GPU will lead to profitable mining. That said, because X11 ASIC rigs are not on the marketplace, it has become harder to profitably mine X11 with a home mining rig.
Nonetheless, X11 remains more energy efficient to mine and ASIC mining rigs are not yet pervasive in mining X11. On the other hand, mining X11 still requires a tricky-to-build mining rig while X11 also restricts you to one popular coin, Dash. The other X11 coins are really not very prominent.