I'm confused by how preference categories work. Is there any logic behind how the various categories are set up?
Skills, experience, etc... limited number of visas per year.
This is a question for Immigration 101 in school. You can find answers on www.uscis.gov.
There is logic behind the categories. The Visa Bulletin lists each category and explains then briefly. If you have a specific questions such as "I have a bachelor's degree plus 5 years of industry experience, do I qualify for the Employment Based 2nd Preference category." you should post that more specific question which may be easier to answer. Be certain to include all relevant information.