It depends a lot on the kind of traffic you expect and the condition of the ground. I'm going to write from my experience, outside Seattle, WA, USA, where the subsoil is glacial till.
For light foot traffic, pea gravel can be nice. Rocks are under 1", uniform in size, and have smooth surfaces. Every time you step on it, the rocks will shift, softening the footfall. This is also noisy and causes the gravel to spread out. You can contain it with edging, or just let it spread and refresh it periodically. It's not good for wheeled traffic, as the wheel will just sink in.
If you want to push a heavy wheelbarrow, ride a bicycle, or drive on gravel, you want to pick something with sharp edges. It also helps if the rocks are a mix of sizes. They will lock together over time, providing a sturdy surface.
If the ground is soft and collects water during your rainy season, smaller gravel will just get pushed in to the mud with traffic. An initial layer of larger rock (say, 3" minus) will stabilize things, and a top layer (say 3/4" minus) will give a smooth surface. Mud will still work its way to the surface, but the ground will remain stable. In very wet soft ground, I've heard that a layer of 3" - 5" crushed rock works well under the 3" minus, but I haven't tried it.
To keep mud from working its way up, put a layer of landscape fabric down first.
Look at the shape of the land and the history of draining. Avoid low, wet areas, or reshape them to drain elsewhere. Consider how your traffic will wear and shift the ground, and how that will affect draining in the future, too (e.g. tires leaving ruts).
It a good idea to remove topsoil and other organic matter first. Keep it, though, and use it elsewhere.
Do you need? No. You could technically lay them down on grass and they'd probably be OK for a short period of time. But the next person who rents the house will be annoyed, and so might the landlord when the pavers settle.
A 30kg of gravel is about $5 at my local HD - so how much money are you really saving here? And lets say you save $50 - if they settle while you are still living there, it will cost you more than $50 to fix after-the-fact.
Do it right the first time.
Best Answer
I can't answer from a building perspective, but I from what I recall (working from memories over a decade old) from my geotechnical engineering classes:
Then you get to silt and clay at smaller sizes. I might be wrong on the boundary between rock & gravel, as the wikipedia page for gravel lists the upper limit o gravel at 64mm / 2.5 inches, and the lower limit of sand lower as well.
You also see terms like 'coarse aggregate' and 'fine aggregate' when dealing with concrete mixing (fine tends to be sand or rock dust, coarse is larger).
You've already seen the 'smooth' (aka 'river rock', aka 'pea gravel' when small) vs. 'crushed' (aka 'sharp') issue. Smooth will always leave voids and won't compact well, so I'd assume it'd be better in drainage situations
There are also products available that are of mixed grade for better compaction (I've beard it called 'crusher base' or 'crusher run'), which are intended for compacting before your sand course for patios and the like. In some areas, you might also be able to find recycled crushed concrete.
I'm not going to get into larger rock products, as there's just too many (flagstone, slabs, blocks (dimensional stone), tumbled blocks, cobbles and pavers, riprap, decorative boulders, etc.)