For family based petitions, each category is based on the type of relationship the U.S. citizen or green card holder petitioner has with the beneficiary.  

Parents, spouses and children under 21 of U.S. citizens always have a visa number available so they will not have to wait.  

Everyone else must fit into one of the other visa categories.

Here are the official definitions of each visa category.  

FAMILY-SPONSORED PREFERENCES

First: (F1) Unmarried Sons and Daughters of U.S. Citizens:  23,400 plus any numbers not required for fourth preference.

Second: Spouses and Children, and Unmarried Sons and Daughters of Permanent Residents:  114,200, plus the number (if any) by which the worldwide family preference level exceeds 226,000, plus any unused first preference numbers:

1. (F2A) Spouses and Children of Permanent Residents:  77% of the overall second preference limitation, of which 75% are exempt from the per-country limit;
2. (F2B) Unmarried Sons and Daughters (21 years of age or older) of Permanent Residents:  23% of the overall second preference limitation.

Third: (F3) Married Sons and Daughters of U.S. Citizens:  23,400, plus any numbers not required by first and second preferences.

Fourth: (F4) Brothers and Sisters of Adult U.S. Citizens:  65,000, plus any numbers not required by first three preferences.