Immediate family = your close family, usually parents, siblings, husband/wife, children. This doesn't count uncles, cousins, etc. Think of "immediate family" as the family members who live/lived with you. Family reunion = a party with your family, where you might invite your immediate family and extended family to socialise and have fun. Usually a family reunion is held to see family you haven't seen for a long time.

Your immediate family includes only the people that are within your home, such as your mother, father, sisters and brothers when you are young, or your wife or husband and children when you are older. Your extended family includes everyone that is related to you such as your immediate family plus your grandparents, aunts and uncles, cousins, nieces and nephews, and even your distant cousins, great grandparents, great aunts and uncles, and great nieces or nephews. A family reunion is a gathering as many of the extended family as are able to attend. This is usually a very large party where everyone comes together to spend an entire day or weekend.

Immediate family are parents, children siblings (brothers/sisters) it also refers to grandparents or grandchildren. Extended family are uncles, aunts, cousins, nephews. Even very close friends sometimes are considered extended family.