This is as my friends have said, a sourdough sarter. Mine is from a cullture that has been active since 1847. Mine comes from the same source, and has been active for ten years now.
If you would like a great sourdogh starter I would suggest the Carl Griffith 1847 strain. If you send them $5 for postage and maintenance there are people that have as a testament to Mr. Griffith, have been preserving his starter for years.
the friends of Carl Griffith have a website here:
Carl Griffith 1847 Oregon Trail Sourdough Page
A sourdough starter is different than normal yeast, in that it is a start to a dough making process. The yeast in it is something that can last for decades and centuries, just involves being fed a mix of 2/3 flour to 1/3 water once a week, the yeast will live. It does tend to adapt itself to your local yeast and bacterial situation. When I moved to Oklahoma, and when I moved back to PA in both cases it took the yeasties a month to adjust, they made bread, it just wasn't as good as it normally was.
I keep mine in a porcelain pot from King Arthur Flour, feed it once a week or so. Mine has been alive now for 16 years.
Commercial yeast is asleep, and dies after a while, a sourdough starter is a living colony of yeast. Thus it refreshes itself. It does take some maintenance, and has to be fed about once a week or so (you can fudge this). If you want to start a starter, helps to have a dedicated, not airtight container you will use for it.
You want to pull it out once a week, feed it, and let it go back to sleep, if you aren't using it. Use it often and you are just talking of replenishment.