Broadly, seeds fall into three camps: Heirloom, hybrid or GMO (Genetically Modified Organisms). So what's the difference?
Heirloom seeds are seed that reproduce "true." That is, if you save the seed from a Heirloom variety, when planted it will produce a plant (and fruit) that are exactly like the parent plant. As a result, Heirloom seeds are often passed down through generations from gardener to gardener (hence the "Heirloom" name). I save seeds every year from my favorite tomatoes and squash, and those become my plants for the next year.
Hybrid seeds are seeds that are created from crossing two types of plants. They are, for example, the seeds that would occur from crossing a Brandywine tomato with a grape tomato (would that create a "wine-grape" tomato plant?). This happens in nature all the time (it's how nature creates new varieties!), and companies that create and sell Hybrid seeds just do this on a mass scale. The benefits of a Hybrid are creating new plants that have strengths from both parents. You could, for example create a tomato that had the wonderful flavor of a Brandywine with the higher output and greater disease resistance of another variety. The problem with Hybrids is that they often don't breed true. That is, the seeds of the plants grown (the seeds of our "wine-grape" for example) do not usually create plants that produce hold all the characteristics of the parent. In fact you're never quite sure what you might get (my compost pile has produced some pretty strange looking "volunteer" squashes over the years). So with Hybrids, you can't save seeds from season to season and know what you're going to grow. Many people believe the greater disease resistance and bigger yeilds of some Hybrids outweigh the inconvenience of needing to buy seed every year.
GMO seeds literally have parts of their DNA strings removed by scientists and replaced with strings of DNA from other plants (and even animals) to create entirely new, and usually patented, living things. GMO seeds have often been created for specific purposes such as bigger yields, disease resistance, or ability to thrive when sprayed with pesticides. There is a lot of controversy around the world as to the safety and wisdom of creating entirely new life forms and setting them loose to breed on the planet. While a GMO may reproduce true like an heirloom seed, because they are usually patented it is often illegal to re-plant the seeds of GMO plants. You need to buy them each year from the company that creates them. Many large scale growers find that the economic benefits of diesase and pesticide resistance and greater yields of GMO seeds outweight the potential risks.
Many AeroGrow seeds are Heirloom varieties, and we source Heirlooms whenever possible. We do use Hybrids when a specific plant has benefits in an indoor garden (remember a Hybrid is just a seed that had parents from different varieties). AeroGrow never sources or uses GMO seeds.