I hinted back in my first post about seed saving that you need to ensure you don't have crossing between plants of the same genus and species (intraspecific crossing) in order to maintain genetic purity. This means that you need to learn the genus and species of any plant you want to save seed from, and make sure nothing else you are allowing to flower at the same time can cross with it.
The thing is, it's also possible for some crossing to occur between plants that are the same genus but different species (interspecific crossing). I won't discuss the particulars, but basically some plants within a genus that are different species are more closely related to each other than others. This allows for crossing to occur on occasion. To make sure interspecific crossing isn't occurring, you will need to do a little more research or isolate your crops well.
A paper published in 2006 by plant breeder Jim Myers of Oregon State University discusses the crossing of canola with various other brassica's of the same and different species'. It is well worth the read if you are interested in how this works. Get it here.
For the backyard gardener crossing may pose little to no concern. If you are saving seed and don't care if broccoli and kale cross, so be it. For someone interested in swapping seeds with neighbors or others through informal seed networks this is something to consider. Most people involved in seed networks have a reasonable expectation of genetic purity.
Saving seeds is very fun and rewarding, but it does require a little bit of research on the part of the saver. Hopefully this post and the three previous bring you a little closer to becoming a successful seed saver.