Cardano is one of the most important and interesting projects on the market. Founded by the co-founder of Ethereum, Charles Hoskinson, the cryptocurrency in 2017 had a sensational growth that stopped in January, following the general collapse of the market. It currently boasts the ninth position in the global market capitalization with a capitalization of about 2.7 billion dollars.
Charles Hoskinson called Cardano a "third generation blockchain". He believes that the cryptocurrencies used for the simple transfer of money are first generation blockchains, an example is Bitcoin. The second generation blockchain can be that of Ethereum because it introduces smart contracts, decentralized apps and is more than just the transfer of value. It also has problems (scalability, lack of governance, etc.).
Cardano is designed to solve both the governance problem (which led to the crossroads from Ethereum to Ethereum Classic due to DAO hack) and scalability. The Proof of Work (PoW) is certainly a proven mechanism, but many think it is not the best and over the years several alternatives have emerged.
Cardano uses a particular type of consensus mechanism called Proof of Stake called Ouroboros. 
Scalability of Blockchain, Cardano and Ouroboros
Ouroboros not only solves the problem of scalability but also the number of transactions per second and the storage of data. This last problem should not be underestimated. With an increase in the use of the network, the blockchain begins to be heavy. Remember that transaction data must be propagated to all participating nodes. To solve this problem, Cardano is trying to implement RINA (Recorsive Inter-Network Architecture) technology to improve communication speed.
In addition, for a blockchain platform that offers the possibility to create smart contracts and decentralized applications, the size of the stored data can grow very rapidly.
Bitcoin's blockchain, for example, is already over a hundred gigabytes and will continue to grow with the extraction of new blocks. Cardano's approach to combat this problem consists of the implementation of sidechain and the use of data compression.
Ouroboros Vs. Ethereum PoS
First of all, it must be said that Buterin and his associates are working to bring Ethereum to the PoS. Ethereum should soon switch to Casper by adopting sharding. One of the fundamental advantages of Ouroboros on Casper is its academic background and the solid mathematical evidence that has been made available to the public.
Despite the same Vitalik Buterin is co-author of Casper FFG paper and mini-spec FFG, according to the developers of Cardano these documents do not sufficiently describe all that is behind Casper. Furthermore, Ouroboros has been thought since the beginning as a mechanism for a PoS blockchain, something that can not be said of Casper's initial specifications. This means that the Ethereum blockchain could be vulnerable to a 51% attack even after the migration to Casper.
There are also many other differences between the two protocols: from security to the way a transaction is executed.
In essence, the Ouroboros protocol works on the basis of the honest majority. The hypothesis is that any majority shareholder (hence a person with an important amount of money) is likely to be honest, since he has no reason to see the value of the cryptocurrency crash.
For approval on Casper, instead, 2/3 of all validators are mandatory. Buterin said that the system strongly encourages the validators not to try to be intelligent because if the system finds them they lose their reward
Ouroboros and Casper also have some things in common. For example, in both mechanisms the older blocks are very difficult to reverse.
It is clear that Ouroboros is not infallible and much less perfect, but probably no mechanism of PoS is today. Vitalik Buterin and Prof. Aggelos Kiayias have extensively discussed the different implementations of the two technologies. In the end, however, both are trying to reach a similar goal, which is no longer worrying about the scalability of the blockchain.