From a genealogist's point of view, an uncle is the brother of your mother or father1. The (non-blood relative) spouse of your aunt is also called your uncle, though more correctly should be called your uncle-in-law (which is also what you would call the uncle of your spouse). If you have an uncle, then you are their nephew (male) or niece (female). While it is most common for your uncle to be older than you, it is well within the realm of possibility for you to be older than your uncle if your grandparents were still having children after you were born. Like cousins, uncles are neither ancestors nor descendants of you, but are simply related to you through other branches of your family tree.

Despite the formal definitions, many cultures throughout the world use the word uncle to mean various things. Step-fathers and basically any sort of secondary or replacement father figure have been referred to as uncles in various cultures. Quite often this was brought about through the custom of having a man marry his brother's widow. The children would continue to call their mother's new husband their uncle (because that's what he was). If there was no brother, or the brother could not marry the widow, it simply became customary for the children to call any new father their uncle. In addition, some cultures would call older male cousins uncles (though it is not as common to refer to an older female cousin as aunt). Some children were often encouraged to call close friends of their parents "uncle" as well.

Grand / Great Uncles

If the brother of your parent is your uncle, what is the brother of your grandmother or grandfather? This relative is called a grand uncle or great uncle. Genealogy purists like the term grand uncle best because it fits well with the nomenclature for your direct ancestors - grand uncle goes with your grandparent. Similarly, your great-grand uncle is the brother of your great-grand parent. Each generation you go back adds another "great" at the beginning of the name of the relation. In contrast, if you go down the family tree instead of up it, you become the uncle, grand uncle, and great-grand uncle and your relative becomes your nephew/niece, grand nephew/niece, and great-grand nephew/niece.

Uncles Diagram

To help you visualize the relationships, below is an ASCII diagram. All relationships are based on relationships through the male relative for consistency. Only blood relatives are shown.

                     |
                  +--+--+
                  |     |
          GGM--+--GGF   GGU   Great-Grand Mother, G-GFather, G-G Uncle
               |
            +--+--+
            |     |
     GM--+--GF    GU          Grandmother, Grandfather, Grand Uncle
         |
      +--+--+
      |     |
M--+--F     U                 Mother, Father, Uncle
   |
   Y                          You

Note to reader: All information can be applied to the aunt relationship simply by replacing all masculine references with feminine references.

1 Or alternatively, the son of your grandparents who is not your father.