For employment based petitions, each category is based on the type of employment you are accepting.The visa categories are usually based on skill and experience and industry.Each visa category has a certain number of visas available every year. If the U.S. government receives more applications than the number of visas available, then there is a backlog and a wait develops.

Here are the official definitions of each visa category. Please remember that this is a very confusing chart and is not that helpful. Each visa category will have its own technical requirements. Generally speaking the more highly skilled the position you are seeking is, the faster the visa number becomes available and the faster you will get through the process.

EMPLOYMENT-BASED PREFERENCES

First:  Priority Workers:  28.6% of the worldwide employment-based preference level, plus any numbers not required for fourth and fifth preferences.
Second:  Members of the Professions Holding Advanced Degrees or Persons of Exceptional Ability:28.6% of the worldwide employment-based preference level, plus any numbers not required by first preference.
Third:  Skilled Workers, Professionals, and Other Workers:  28.6% of the worldwide level, plus any numbers not required by first and second preferences, not more than 10,000 of which to “*Other Workers”.
Fourth:  Certain Special Immigrants:  7.1% of the worldwide level.
Fifth:  Employment Creation:  7.1% of the worldwide level, not less than 3,000 of which reserved for investors in a targeted rural or high-unemployment area, and 3,000 set aside for investors in regional centers by Sec. 610 of Pub. L. 102-395.