My local bicycle shop is offering "premium kevlar belt tyres" and are "extremely puncture proof" how much of this is true? Does this "kevlar belt" offer good puncture protection? What does this protect against? Shards of glass? Stones?
Kevlar fibres do resist cutting, and the kevlar belts in my tyres have certainly stopped lots of sharps (mostly glass and flint).
What it won't protect against are pinch flats (which you can avoid anyway by keeping your tyres at the right pressure) and, if it's just a central belt, it may not protect against sidewall cuts either.
There's generally a tradeoff between ride quality and puncture resistance FWIW: my tyres are lighter and faster than Schwalbe Marathon Plus (for example), but also less tough.
If you want anything more than generalities, you'll need to name those tyres and ask about them specifically.
Beware of tyres claimed on-line as having a Kevlar Belt in them. They may have just Kevlar particles blended into the tread compound to reduce wear. Maxxis Refuse is a tyre that has K2 on the tyre sidewall and a statement on the packaging/material/label that comes with the tyre as stating a Kevlar Puncture resistant Belt. I have been using Vittoria tyres for years and after trying to get some of the same quality tyres from Vittoria I have had to change to Maxxis Refuse.