Kitchen

[Campion]

The method you choose isn't "old fashioned" or not--it's whether you have high acid, high sugar foods or non-acid. For example, tomatoes are acidic and can be canned with the hot water (boiling water bath) method, where you sterilize your jars, put in the tomatoes and juice, and boil them with lids on. 

The pressure method is for foods that are not high sugar or acid. In a low acid, low sugar environment, spores like clostridia botulinum grow even in a sealed, heated jar and make the famous food poisioning botulism. So to break the spores and kill them, you need higher temperature than you can get by boiling, hence you use a pressure cooker at a specific pressure to reach the right temp that breaks the spores. 

The best book I know of is Simply Canning. It goes over the foods you should pressure can and how to do it safely.