There are road bike forks and mountain bike forks, for road bikes you can have steel, alloy or carbon forks and they can be straight or with a rake, which is a bend in the fork, the straight and the raked forks do the same thing, but the rake can give more of a comfortable ride.
The differences between steel, alloy and carbon is mostly price and weight. On most bikes now, the fork of choice is carbon, some have alloy steering column, but if you want the best it has to be a full carbon set, with carbon steerer.
For mountain bikes you have to get suspension forks, the original suspension forks had a spring in each arm.
You could use different spring for different weight riders and surface conditions, but they were very basic. Forks evolved to have oil or air damping, which is adjustable and you could have different rebound. All very complicated but you could ride over any conditions and in all weathers.
There are many suspension forks on the market, probably the best known is Rock Shox, but there is also Manitou, Marzocchi, RST and Girvin.
You have to buy forks with a steering column depending on which kind and size of headset you have.
There are different size columns, 1 inch, 1 1/8inch and there is occasionally 1 1/4 inch, and it has to be threaded or smooth for standard or aheadset.
Forks are very important, as you really need them to steer your bike in the direction you want.
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