Where can I find a description (preferably a diagram like below) showing the block structure in an Ethereum blockchain?
Lets first start with my adaptation of this figure illustrating the Bitcoin block structure:
So far so good. All fairly basic.
The header field definitions are available in section 4.3 of the yellow paper.
The description can be found in the Yellow Paper which is the formal specification of the Ethereum protocol.
Here are the main pieces of a block:
4.3. The Block. The block in Ethereum is the collection of relevant pieces of information (known as the block header), H, together with information corresponding to the comprised transactions, T, and a set of other block headers U that are known to have a parent equal to the present block’s parent’s parent.
No diagram though (an opportunity for the community).
Here's a picture from V's article regarding data structures for light clients:
Vitalik Buterin gives a compact answer:
Every block header in Ethereum contains 3 trees for three kinds of objects: Transactions, Receipts (essentially, pieces of data showing the effect of each transaction), State.
You can read a longer version with pictures here.