What is the difference between an Immediate Relative petition and a Preference petition?

What is the difference between an Immediate Relative petition and a Preference petition?

Immediate Relative petitions are those filed by US citizens for their spouses, children (below 21), or parents. Preference petitions are those filed by US citizens for their sons or daughters (21 or above), or their brothers or sisters. Green card holders may also file for their spouses, children (below 21), or unmarried sons or daughters (21 or above). All other relatives not mentioned (for example: grandparents, grandchildren, uncles and aunts, nephews and nieces, cousins) are not eligible to be petitioned for a family-based visa.