Here is a really good visual explanation of different types of SQL joins. I was recently asked what the difference was, and while I knew the answer, I fumbled to articulate it. I should have simply said “An inner joint results in a subset or overlap of results and an outer join results in a union of results from two or more tables.” (i.e., outer joins return records for the left side even if nothing exists for the right side.)

[Coding Horror: A Visual Explanation of SQL Joins]