H1A visas are for registered nurses.
H2A visas are for seasonal or temporary agricultural workers.
H1B visas are for specific professional jobs and are specific to a company and are supposed to be temporary. They can be transferred from company to company, but there's supposed to be a six year maximum. In theory, it's hard to transfer from H1B to green card, but it happens more often than not.
H2B visas are for non-agricultural, non-technical workers for temporary (1 year or less) or seasonal work. Employers' needs cannot be ongoing or continuous.
Regarding H2A visas:
USCIS application procedures :
H2-A Temporary Agricultural Workers
The H-2A labor certification program was created as a mean for agricultural employers who anticipate a shortage of domestic workers to bring nonimmigrant foreign workers to the U.S.
In order to obtain an H-2A certification, the employer must show to the DOL that his need for a worker is seasonal or of a temporary nature. "Temporary or seasonal nature" means employment performed at certain seasons of the year, usually in relation to the production and/or harvesting of a crop, or for a limited time period of less than one year when an employer can show that the need for the foreign workers(s) is truly temporary.
Second, the employer must also demonstrate that there are not sufficient workers who are able, willing, qualified, and available and that the hiring of a temporary foreign worker will have no adverse effect on the wages or working conditions of similarly employed US workers. That wage or rate of pay must be the same for U.S. workers and H-2A workers.
Employers are required to provide H-2A workers with the following benefits:
The employer must provide free housing to all workers who are not able to return to their residences the same day,
Three meals a day to each worker or furnish free and convenient cooking and kitchen facilities for workers to prepare their own meals.
The employer must also provide free transportation to the work site.
In terms of insurance, workers compensation insurance will be granted where it is required by state law.
The H-2A certification is valid for up to 364 days. Extensions of up to one year are possible, but cannot exceed three years.
The reason illegal labor gets used instead of the H2A program is that the H2A program is expensive for employers. (Housing, transportation, and market level wages?? Oh no!)