The Mandela Effect is the difference between the recollection of a large portion of the population and what written records show actually happened. These come in lots of variety.
1. Movie Quotes
2. Historical Changes
3. Religious Changes
4. Geographical Changes
5. Changes to Physical Products
Here's a short quiz that will show you some of the effects you may have been exposed to and help determine if you are originally from this exact universe or somehow it's different.