With the assistance of rssslvr, from Calguns
Punches are tools which consist of a fairly wide striking surface at one end, and a narrower surface on the contact end. They can be made of a variety of materials, depending on whether the desire is to move something, destroy something, or mark something.
Types of Punches
This is by no means all-encompassing, but these are the more common punches one will encounter.
Chisels - while not exactly a punch, most large punch sets do include a few chisels.
Pin punch - flat on one end. This is the most common type of punch in metalworking.
Pointed punches - as implied, there's a point on the contact end. These punches are typically used to dent or mark a surface.
Roll pin punches - mostly flat, with a small bump in the center to help keep a roll pin from slipping during installation.
In order to prevent damaging or leaving marks on the surface, punches may be designed out of a softer material. Common alternative materials are brass or delrin. Most punches are made out of steel, however.
Depending on what you're trying to do, you may find yourself breaking punches. It is important to note that not all punches are created equal! Avoid all 'cast' punches, when working on metal - they are structurally weaker, and will readily shear off. It's easy to injure yourself when a cast punch snaps, as the fractured end tends to be sharp and can easily be driven into your hand.
Punches to Avoid
Cast Punches: Punches which are made from cast steel are far more fragile than forged punches. When looking at punches in a store, check to see if they're cast. If they are, they are unlikely to stand up to anything more than very occasional metal work.
Dasco Brand: The ubiquitous blue-handled Dasco punches are cheap for a reason! They are cast, and shear VERY easily. The price (usually around half that of Craftsman) is a false economy, as you will break 3-4 Dasco punches on gun builds in the time it takes to break a single forged Craftsman punch! Normally I avoid disparaging comments about a given brand, but in the case of Dasco, their fragility constitutes a hazard to anyone working on metal with their punches. That having been said, if you're not planning on doing any serious hammering and other work, a Dasco variety pack isn't too bad of a deal. Just don't ever consider them for AK builds...
Brands to Use
Craftsman: While by no means indestructible, the red-handled forged Craftsman punches are a very high quality consumer-level punch. They will also tend to bend instead of simply shearing like the cast Dasco punches.
Snap-On: Although I haven't used their punches, automotive mechanics frequently claim that Snap-On punches are the most durable out there. Based on the few Snap-On tools I own, I would suspect their allegations are correct.