Hi perennialfan,
A Cultivar is a specific term referring to a form of plant species that is artificially maintained to preserve it's unique characteristics and does not occur naturally without intervention. I am sure Webster's provides a more concise definition, but that is how I think of it.
In order to get exactly the same Cultivar as the parent you started with, you would have to resort to vegetative propagation. For annuals, you would have to duplicate the original crosses in your backyard to get comparable results by perpetuating that Variety.
There are plant Varieties that are true Open Pollinated forms, which means they are largely self-fertile and will reproduce the same results generation after generation with little drift.
You can achieve this condition with some hybrids through a painstaking process called dehybridization.
And yes, if you have more than one variety of the same species, crosses can occur. With some promiscuous plants like Salvias even interspecific crosses happen.
Isolation will help maintain the integrity of a specific variety. The distance involved varies for different plants but I look them up individually when I want to know how far apart to plant different ones.
Especially when it comes to annual flowers, I try not to think about all of this too much. I regularly collect seeds from hybrids and I think a lot of seed traders do as well. Most folks know that the results they get from collected F1's may be surprising. I have to say that I have been pleasantly surprised by how often the offspring closely resemble the parent. A lot depends on how much you value uniformity. Some folks would never use seeds collected from F1 hybrids, but I have enjoyed many of them. I would not use seeds saved from hybrid vegetable varieties because the differences there will be more than cosmetic and may not be readily apparent.
I have noted a marked deterioration in the quality of plants saved from subsequent years. The further away you get from the parent, the less like the original the progeny become.
Whether you are using hybrids or not, there are things you can do to influence the results you get from your saved seeds. Save from several different plants whenever possible. Choose only the best exemplars and choose those that have characteristics you value. You can hand pollinate those that you like best and continue through several generations to establish your own variety.
Just because you don't find it in a seed catalog somewhere does not mean it has no value.

Oops, sorry-I am not usually this long winded. I will stop now. I hope I did not add to your confusion....
Alana

its easier than frog makes it [boy i switched two letters and wondered about that name drog.. lol]

if you want a specific ANNUAL.. and it dies every fall.. you must BUY IT AGAIN ... its a specific cross of genes to make a specific flower ... potential to spend mondo bucks ...

if you want to cover space.. at NO COST ... and dont really care about a specific color scheme.. the garden fairies will provide quite a bounty .. for those that will sprout in spring ...

it is the difference between a cottage style riot of plants.. and Versailles ... with its foo foo gardens ..

so for me.. FREE IS BEST.. and riots are in order ... and a pox on foo foo ...

but whatever rocks your boat.. is just fine with me ...

ken

As others have mentioned -- sowing hybrid seeds is usually a "crap shoot". The combining of genes during reproduction is random which results in different combinations of traits being expressed. And just to make things even more frustrating, there are times that plants, when pollinated, will produce sterile (nonviable/"dead") seed.