Gemination is where a developing tooth splits into two separate teeth. Fusion is where two developing teeth merge into one tooth.
The appearance between the two can be similar, you just have to count the teeth to see if you have an extra one or a missing one. I see a lot of this kind of thing on the top and bottom front baby teeth. It can present a problem if they do not or cannot fall out at the right time and interfere with the eruption of the permanent teeth.
Gemination or Fusion can be total or partial. Here is an photo showing partial-gemination of a baby lateral incisor. You can see this "tooth" has two roots and two crowns but are both joined together, kind of like "siamese" teeth.