When I search online for buying diodes, I find much more zener-diodes than normal diodes. Is there a reason for this? Can you use every zener-diode as a normal diode or is it uncommon to use diodes in circuits and some other circuit is used to replace the stuff you usually would do with a diode?
"Plain" diodes (silicon diodes, germanium diodes, etc) are very common. As Gerben mentions in his rather good analogy, with Zener diodes you have a wider range of options. The actual number of discrete options for Zener is greater than that of silicon due to the wide range of voltages. However, the quantity of silicon diodes on the market and in use in the world is far greater - you just don't have such a wide choice, since there's no need for it.
Schottky diodes, too, give a greater selection due to them having more parameters of interest (switching speed, leakage current, forward voltage, etc) that you don't really care about with a boring silicon diode.
To put it another way and use another analogy - imagine walking into a builder's merchant's. There you have piles of similar items:
- Breeze blocks
- Concrete Slabs
Bricks there is only maybe one or two different types. However there are huge piles of those couple of options. People use a lot of them, but there's not much to distinguish one brick from another.
Breeze blocks there may be a number of different sizes, some hollow, some solid, etc. There's less of each type, but there's more piles.
Finally there's the concrete slabs. You have different colours. Different patterns. Different sizes. In short, there's many many many different options. So there's many many different piles. However there's only maybe one pallet of each, compared to twenty pallets of one kind of brick.
So although it may look like there are more Zener diodes around, all there actually is is greater choice of parameters for Zener diodes. There is actually far more silicon diodes around in the world. You'll be sat within a few feet of tens or hundreds of silicon diodes right now, but maybe just a small handful of Zener diodes.
You can get zener diodes in different voltage ratings. So you naturally you get more different zener diodes than normal diodes.
Just like in the supermarket. There are more types of flavored yogurt than plain yogurt. As there is banana-flavored yoghurt, strawberry-flavored yogurt, and so on.
If you just want general-purpose diodes, buy a bunch of 1N4148 diodes for low current and 1N4004 diodes for higher currents, and maybe a handful of 1N5819 Schottky diodes for when forward voltage drop matters. The 1N4004 are 1A 400V PIV diodes, so you can use them at lower voltages and currents. The 1N4148 are at least 150mA/60V, and the 1N5819 are 1A/40V diodes so good for lower voltage circuits.
They're all very cheap generic through-hole parts and any junk box should have a good supply of them.