Alkaline batteries, like almost everything, have a variable shelf life depending on the storage temperature. The current cells have a 3 year shelf life (defined as still having 80% of original capacity after sitting 3 years at 70F), this time can be doubled by having them at refrigerator temperatures. Don’t freeze them, though!
They come in standard sizes: AAA, AA, C and D are the common ones. These all have the same output voltage, 1.5V. There is also a 9V battery that uses 6 small alkaline cells in a common case. Batteries are normally put in series to provide more voltage.
Cell storage capacity and maximum load is by the cell size. The larger the battery is, the more electrons it can hold and give. The remaining capacity of these batteries can be roughly determined by their voltage. 1.5v=most to all left, 1.4v=over half gone, 1.3v=very little left, 1.2v or less = useless. Generally speaking, lower current draw means more total capacity from the battery. For example, the ubiquitous AA cell can last over 6 hours with a 300mA load, but can only handle 45 minutes of a 1000mA (maximum for that cell size) load. My digital camera uses two AA cells, they need to be replaced after an hour of heavy picture-taking. A LED flashlight using the same cells will last 10 hours before needing new batteries.
So, why have these instead of rechargeable batteries? They have the best shelf life by far, so in an emergency when you need a flashlight that works Now, alkaline batteries are your friend. For daily use, they can get expensive quick.