Most people come into the USA on non-immigration visas. For a complete list of non-immigration visas, please visit this page. Among those visas, H2B is a working visa that allows people with very minimum qualifications to come into the United States. H1B and O-1 visas are working visas that require the person on the visa to possess certain qualifications.
If you are already in the US and want to apply for or change to a working visa, H1B, H2B or O-1 visa is the most often used. H1B is for specialty worker, which means you get to have special training or at least an associate degree. It is good up to 3 years and can be extended for another 3 years. There is a cap of 65,000 new visas each fiscal year with certain exceptions. H2B is for seasonal worker, there is no special requirement on the beneficiary (you) except you need to be able to perform the job offered. It is good up to 1 year and can be renewed twice up to a total of 3 years. There is a cap of 66,000 new visas each fiscal year. O-1 visa is for people with Extraordinary Ability, no quota limitation. It is good up to 3 years and can be extended for another year.
The working visas have to be sponsored by a US employer. United States has very liberal business laws. Anyone in the US could be an employer. The requirement on the employer is that the employer has to be able to provide prevailing wage for the alien on the visa.