Forever stamps guarantee delivery forever (subject to terms and conditions on weight/size). Regular stamps denote value, e.g. 50 cents. They will always be usable but you will need to match the inflation of prices (e.g., mailing a letter might be 50 cents in the past but 100 cents in the future)
Numbers for demonstration only
USPS still sells denominated stamps from $0.01 to $10.00, in addition to "First Class Forever" stamps - see near the bottom of the filters sidebar on https://store.usps.com/store/stamps
There's also other types of forever stamps, such as the domestic Postcard and Global Forever stamp!
These are problematic as well - they don’t have a way to tally the stamps, they just look for the marker and process it. A postal employee at the delivery point may flag it.
Also if you mail stuff this way you’ll have mail inspected for bombs or anthrax.
It's common in the UK to sell them both with a £ denominated value on them or the class of service. My brother bought loads of the latter a few years ago, which was a great idea, because as mentioned postal rates have increased significantly. The UK post office will also exchange your old unused stamps for new versions (now with QR codes or with sticky backs) for free. So even if they go out of date they alcan be upgraded at no cost.