For a small building or shed, the pier foundation is perfect - quick to do, and takes the least amount of embodied energy.
What is 'embodied energy'? That's the amount of energy that it takes to produce the items you use.
So for this pier foundation, there is concrete, but it's a much smaller amount than, say, a perimeter foundation where the entire perimeter is formed and poured.
There are two types of pier foundation; footings made from Sonatube (heavy duty cardboard tubes), either with or without a 'Big Foot' at the bottom.
This entails digging a hole, then making sure that the footings are level and plumb.
The second type is to use specially designed concrete blocks that are formed to hold a 4x4" post. These are called 'pier blocks' in my area - maybe do some research on the name they go by in yours.
There is also an alternate type using the same principles, with a saddle made of metal which has a post on the bottom to sit in a hole in the concrete, either poured or in a block, and the post is attached by being screwed in place.
Either type works, depending on your project.
These specially engineered blocks are designed to be used with 4x4 posts, which fit perfectly into the block.
We used these for the Livery woodshed, which needs to hold a lot of weight in the winter. We've had four feet of snow at times.
The design of the block prevents the posts from shifting.
For a lighter project, such as a woodshed or chicken pen, I would use these concrete blocks.
They're cheaper, and easy to install; they just sit on top of your prepared area - a small amount of gravel under them will make sure they don't get water under them and heave with the frost.
The more intensive pier foundation, with the Sonotubes and Big Foots, entails a lot more disturbance to the ground (holes must be dug, preferably with a machine, or hand dug).
The Big Foots have a length of Sonotube attached to them with screws and then the whole thing is put into the prepared holes, then back filled.
This is the method we used for Pearls Place - see information on the foundation here.
For a home or addition to one, a deck, or a large shed or shop, I would recommend the more intensive method.
This will hold a lot more weight without shifting. I've seen foundations where people tried to cheap out, only to find the whole structure put at risk from the foundation failing due to frost. Don't be this person.