Updated: Dec 10, 2022
Yeast is a living single celled organism that burps carbon dioxide gas while it eats sugar. Humans have been utilizing the happy little critters to make their bread fluffy for a really long time; think thousands of years. When dehydrated and kept at the correct temperature, yeast becomes "suspended" and stays viable for long periods of time. When you're creating a dough you "wake up" the yeast and they go right to work eating and multiplying; which in turn rises the dough. Baking the dough kills the yeast after they have done their work.
There are many types of yeast products; I'm specifically covering dry granulated yeast in this post. You can purchase dry yeast in packets, small jars, or bulk vacuum packages. Whatever size you purchase, keeping the yeast viable until you are going to use it is quite important. I buy larger packages and place the contents into half pint mason jars, add lids and rings and freeze. That way the current jar I'm using is the only one exposed to any warmer temperature (only for a minute at a time) and the remainder of the yeast stays frozen. This will allow it to stay useful for years.
You can store a jar of yeast inside the cabinet in the kitchen but the warmer temps above freezing allow some of the yeast to die. Eventually, all the yeast die and when you go to make bread, it won't rise. Once you get into the habit of grabbing the jar out of the freezer, dipping out the amount of yeast you need and then putting the jar back immediately, it isn't a bother.
Some recipes call for instant yeast or fast-rising yeast. This just means that it will make the dough rise faster. If all you have on hand is regular yeast, you can substitute it in the recipe. I usually add half to a full teaspoon extra of regular yeast to account for the "instant" action.
There are yeast cakes and other types of yeast products available, some are even refrigerated. Sourdough has also increased in popularity. I'm not familiar with those yet and will let other experts educate about those items.