It seems that more than a few people have no idea what the difference is. Is there a difference? Oh yes, there is.
"Heirloom" to identify a cultivar is somewhat ambiguous. By that I mean there is no set perimeters by which to formerly classify an open pollinated strain as "old". As it is all "heirloom" cultivars are open pollinated and often a story can be linked to the name of the variety. Many of these strains were created by backyard gardeners through selecting specific qualities . Some were handed down to the next generation as a family tradition, some were sold to seed companies, etc.
It is like this... you have a horse and it is an Arabian... you breed it to another Arabian.. the result is the foal as well will be an Arabian. Heirloom... the Arab has papers.. you can trace the pedigree to some extent.
Open pollinated strains are not hybrids. A cultivar must be stabilized over 7+ generations to be considered an open pollinated strain. Here is a key difference... open pollinated strains are more current and as well tend to have more focus on resistances. This does not mean GMO, this means by selection. Example.. going back to wild sources to harness a sport that has shown natural means to resist some diseases/ pests/ etc. and then crossing it to a current strain.. then repeatedly selecting the specific qualities until the end result is a stabilized strain with the desired qualities.
I am all for maintaining strains. I find it a worthwhile and rewarding endeavor. In effect by maintaining a cultivar over the years, you are in effect creating your own strain. Unavoidable as over time the plants that do best, produce best, etc. are ones that are becoming acclimated to your micro climate.
What makes me cringe is encountering those that say "only heirlooms!" and don't really know what it means. Old strains are around for a reason.. usually for unique characteristics or superb flavor. Their primary selling point ironically is that they used to be more available... were thought to be almost lost and now are commercially available again. Many of these old strains though are notoriously finicky (and some have little disease resistances), and while some are renown for their flavor, not usually for production.
So if you looking to save seeds, for whatever your reason... personal enjoyment, the challenge, survival preparedness, etc. remember diversity and variety is important. (Then make sure to brush up on how to maintain a strain and control cross pollination!)