Fair question for new shooters. I'll try to sum up common sighting systems.
Iron sights/open sights - mechanical sights you find on a lot of firearms. Usually a vertical post towards the muzzle that you line the top of with a V or U notch or an aperture "hole" sight. Plenty of older guns have them. They're light weight, low profile, and if the gun has them built in, you don't necessarily need to spend more money on a scope or red dot. IMO everyone should practice and get good with irons. Great for close to medium range shooting or hunting.
Bead sight - another mechanical sight you typically find on shotguns. They're at the muzzle end of long shotgun barrels. It's not really a "sight" as much as a guide. You can't get a faster and better sight for hitting a flying bird or clay than with this. Yes, you should aim a shotgun.
Red dot - An optic where a laser makes the appearance of a floating dot or reticle against glass when you look through it. You can mount them anywhere you'd like because they have effectively infinite eye relief. Quick, lightweight, and compact (especially micro red dots), they are nearly always a better option than iron sights these days. They don't provide any magnification. Very popular for tactical or competitive use because of the ease and speed that these sights can be used.
Scope - A telescopic sight that magnifies whatever you're looking at. Endless choices of reticle, magnification, and price point. You better do more research on scopes because there's so much to them. They can get heavy and large for tactical or long range scopes, or light and compact for hunting scopes. They're probably the most versatile sight for most rifle uses. Very useful for long range, precision, and hunting. You can get low power scopes that are like red dots with a learning curve but you can increase the magnification if you need to. One thing though; think about what you want to use your rifle for AND THEN pick a scope based on your needs. It's easier to sell a used gun than a used scope.
