What is an Heirloom Seed?

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One very small part of the class I attended yesterday on sustainable agriculture talked about the differences in seeds.  I have always been a fan of heirlooms because the idea of saving seeds appealed to me.  MrLivingOurWay was always a fan of hyrid seeds because he liked for this seeds to be disease resistant.  After the discussion in class yesterday, I decided to look more into this topic.

According our instructor, an heirloom seed is a variety of seed identified as a named variety in a catalog before 1947.  These have been considered by many to produce a better quality "fruit"  when considering taste, color, etc.

An open pollinated seed is a "regular seed."

Hybrid seeds are seeds that have been improved upon. It is important to note that hybrid seeds are cross pollinated between two different plants of the same species.  This might be a hybrid between two varieties of heirloom tomatoes.  The plants would probably be more disease resistant.

GMO seeds are seeds which have been genetically modified.  I never really understood what could be so wrong with this until I read a little more.  According to Underwood Gardens , not only can a plant be genetically modified to develope it's own herbicide in it's roots but the DNA for the modification is taken from animals and bacteria.  There's just something about eating a hybrid between a fish and a vegetable that doesn't seem quite right to me.

In the end, our instructor suggested that it was best to only plant half a crop with saved seeds and half with new seeds because the saved seeds after years go down in quality.  I'm not sure how I feel about that. How could we still have heirloom varieties if their quality is always going down?  What do you think?