Cloud mining is in which you purchase a share of a mining farm owned by that company and the company pays you in bitcoin according to how many shares you own. They're are many algorithms for crypto but most are either sha-256 (bitcoin) and scrypt (most altcoins). Sha-256 power is usually sold in the form of GH's while scrypt power is sold in MH's. If you want you can rent mining equipment but it is highly unprofitable unless you know what coin is going to go up in value soon. Mining in general is unprofitable unless you own a database, which these companies do. The most reliable/ profitable company at the moment that is offering hashing power is hashnest. hashnest is owned by bitmain which is known for making many high quality miners, they are based in China. I use many sites for comparing ROI at many sites, you can pm me for it as i dont have enough rep to post it here. But keep in mind to do your research on a company before investing, a LOT of companies are ponzis, if your going to invest make sure the company has valid contact info and pictures of their miners or else it is a no go.