Planting a Tuber vs Seed
What is a dahlia tuber?
A tuber is a "structure" similar to a bulb or root that contains energy for a plant to grow. They are similar in shape/consistency to a potato; I have not tried them but they are edible. The latin meaning of tuber is "lump" which is what they look like.
Tuber vs. Seed:
When planted, each tuber will produce a plant that bears flowers. When pollinated, each flower will drop its petals and dry up into a seed pod. Under the soil, the individual tuber that was planted will multiply into an entire clump of tubers. This clump can then be dug in the fall after the first hard frost, and divided into even more tubers for planting the following spring (if stored correctly). See image below for reference.
A seed and a tuber will both grow a plant. Planting an individual tuber will grow an exact clone of the parent and produce multiple other tubers in the fall (that are also clones). Where a seed will produce a hybrid of the parent plant and the pollinator plant. More often than not, a dahlia planted from seed will only produce one tuber for the following year.
[Image: Individual Dahlia tuber divided from a dahlia clump]