Bolts (expansion or glue-ins) are heavily used for protection but the use is controversial in places. They are basically always left in permanently. Push here as been to limit use to areas where there's no other options and to place them carefully to minimize impact. There's a push to only use high-quality stainless (or Titanium for areas exposed to salt) for longest possible life.
The rest of the protections used, nuts, hexes, cams, ice screws, snow pickets, and some exotic devices are usually meant to be removed after use and minimize impact on rock. Although if someone needs to bail in a hurry, leaving expensive gear behind is normal.
Nuts and hexes are basically blocks of aluminium on a wire or webbing sling. Nuts being smaller solid chunks and hexes being larger and hollow. They have tapered shapes (sometime complex, curved, or even designed for a camming action) and you try to wedge them in place. They can be very secure in the right placement. After a good fall on one, it sometimes happen that it cannot be dislodged and it stays fixed for a few months or years.
Cams are mechanical devices designed to grip both sides of a crack and expand outward when loaded. They have two spiral shaped lobes to accomplish this. Very strong when well placed and they have a range of sizes to fit different cracks from tiny body weight only cams around for cracks of 1/4" or so to monsters that fit in cracks of 6" or more.
Ice screws are hollow threaded tubes, the article here does a good job showing how they function.
Snow pickets are T-shaped beams, about 2 feet in length or more that can be driven into hard snow or buried in a trench. Not that strong but workable.
There's a lot of special purpose "protection" that is usually barely able to hold body weight that gets used for aid climbing (when you use the gear to make progress) in situations where nothing else will work. Often to link sections (called pitches) of unaided climbing (called "free" climbing). Free climbing still uses protection in most case, unroped "free-solo" being the exception.