Both are made of the same material. Manufacturers use transparent polypropylene sheets to make aligners and retainers. To adapt them to you, your orthodontist will take measurements, x-rays, photographs, or impressions of your teeth. Invisalign aligners, like retainers, are custom-made to fit your teeth.
However, instead of being made to fit your teeth perfectly, they are designed to apply pressure to specific teeth and move them smoothly little by little. While Invisalign aligners look like a removable retainer, there are many differences between aligners and removable retainers.